BACKGROUND: Peritoneal dialysis (PD) and haemodialysis (HD) are two possible modalities for people with kidney failure commencing dialysis. Only a few randomised controlled trials (RCTs) have evaluated PD versus HD. The benefits and harms of the two modalities remain uncertain. This review includes both RCTs and non-randomised studies of interventions (NRSIs). OBJECTIVES: To evaluate the benefits and harms of PD, compared to HD, in people with kidney failure initiating dialysis. SEARCH METHODS: We searched the Cochrane Kidney and Transplant Register of Studies from 2000 to June 2024 using search terms relevant to this review. Studies in the Register were identified through searches of CENTRAL, MEDLINE, and EMBASE, conference proceedings, the International Clinical Trials Registry Platform (ICTRP) Search Portal, and ClinicalTrials.gov. MEDLINE and EMBASE were searched for NRSIs from 2000 until 28 March 2023. SELECTION CRITERIA: RCTs and NRSIs evaluating PD compared to HD in people initiating dialysis were eligible. DATA COLLECTION AND ANALYSIS: Two investigators independently assessed if the studies were eligible and then extracted data. Risk of bias was assessed using standard Cochrane methods, and relevant outcomes were extracted for each report. The primary outcome was residual kidney function (RKF). Secondary outcomes included all-cause, cardiovascular and infection-related death, infection, cardiovascular disease, hospitalisation, technique survival, life participation and fatigue. MAIN RESULTS: A total of 153 reports of 84 studies (2 RCTs, 82 NRSIs) were included. Studies varied widely in design (small single-centre studies to international registry analyses) and in the included populations (broad inclusion criteria versus restricted to more specific participants). Additionally, treatment delivery (e.g. automated versus continuous ambulatory PD, HD with catheter versus arteriovenous fistula or graft, in-centre versus home HD) and duration of follow-up varied widely. The two included RCTs were deemed to be at high risk of bias in terms of blinding participants and personnel and blinding outcome assessment for outcomes pertaining to quality of life. However, most other criteria were assessed as low risk of bias for both studies. Although the risk of bias (Newcastle-Ottawa Scale) was generally low for most NRSIs, studies were at risk of selection bias and residual confounding due to the constraints of the observational study design. In children, there may be little or no difference between HD and PD on all-cause death (6 studies, 5752 participants: RR 0.81, 95% CI 0.62 to 1.07; I = 28%; low certainty) and cardiovascular death (3 studies, 7073 participants: RR 1.23, 95% CI 0.58 to 2.59; I = 29%; low certainty), and was unclear for infection-related death (4 studies, 7451 participants: RR 0.98, 95% CI 0.39 to 2.46; I = 56%; very low certainty). In adults, compared with HD, PD had an uncertain effect on RKF (mL/min/1.73 m) at six months (2 studies, 146 participants: MD 0.90, 95% CI 0.23 to 3.60; I = 82%; very low certainty), 12 months (3 studies, 606 participants: MD 1.21, 95% CI -0.01 to 2.43; I = 81%; very low certainty) and 24 months (3 studies, 334 participants: MD 0.71, 95% CI -0.02 to 1.48; I = 72%; very low certainty). PD had uncertain effects on residual urine volume at 12 months (3 studies, 253 participants: MD 344.10 mL/day, 95% CI 168.70 to 519.49; I = 69%; very low certainty). PD may reduce the risk of RKF loss (3 studies, 2834 participants: RR 0.55, 95% CI 0.44 to 0.68; I = 17%; low certainty). Compared with HD, PD had uncertain effects on all-cause death (42 studies, 700,093 participants: RR 0.87, 95% CI 0.77 to 0.98; I = 99%; very low certainty). In an analysis restricted to RCTs, PD may reduce the risk of all-cause death (2 studies, 1120 participants: RR 0.53, 95% CI 0.32 to 0.86; I = 0%; moderate certainty). PD had uncertain effects on both cardiovascular (21 studies, 68,492 participants: RR 0.96, 95% CI 0.78 to 1.19; I = 92%) and infection-related death (17 studies, 116,333 participants: RR 0.90, 95% CI 0.57 to 1.42; I = 98%) (both very low certainty). Compared with HD, PD had uncertain effects on the number of patients experiencing bacteraemia/bloodstream infection (2 studies, 2582 participants: RR 0.34, 95% CI 0.10 to 1.18; I = 68%) and the number of patients experiencing infection episodes (3 studies, 277 participants: RR 1.23, 95% CI 0.93 to 1.62; I = 20%) (both very low certainty). PD may reduce the number of bacteraemia/bloodstream infection episodes (2 studies, 2637 participants: RR 0.44, 95% CI 0.27 to 0.71; I = 24%; low certainty). Compared with HD; It is uncertain whether PD reduces the risk of acute myocardial infarction (4 studies, 110,850 participants: RR 0.90, 95% CI 0.74 to 1.10; I = 55%), coronary artery disease (3 studies, 5826 participants: RR 0.95, 95% CI 0.46 to 1.97; I = 62%); ischaemic heart disease (2 studies, 58,374 participants: RR 0.86, 95% CI 0.57 to 1.28; I = 95%), congestive heart failure (3 studies, 49,511 participants: RR 1.10, 95% CI 0.54 to 2.21; I = 89%) and stroke (4 studies, 102,542 participants: RR 0.94, 95% CI 0.90 to 0.99; I = 0%) because of low to very low certainty evidence. Compared with HD, PD had uncertain effects on the number of patients experiencing hospitalisation (4 studies, 3282 participants: RR 0.90, 95% CI 0.62 to 1.30; I = 97%) and all-cause hospitalisation events (4 studies, 42,582 participants: RR 1.02, 95% CI 0.81 to 1.29; I = 91%) (very low certainty). None of the included studies reported specifically on life participation or fatigue. However, two studies evaluated employment. Compared with HD, PD had uncertain effects on employment at one year (2 studies, 593 participants: RR 0.83, 95% CI 0.20 to 3.43; I = 97%; very low certainty). AUTHORS' CONCLUSIONS: The comparative effectiveness of PD and HD on the preservation of RKF, all-cause and cause-specific death risk, the incidence of bacteraemia, other vascular complications (e.g. stroke, cardiovascular events) and patient-reported outcomes (e.g. life participation and fatigue) are uncertain, based on data obtained mostly from NRSIs, as only two RCTs were included.