Impaired cognitive functioning in patients with end-stage renal disease may reduce their capabilities to adhere to complex medical or dietary regimens and to fully participate in medical decisions. With decreasing renal function, cognitive abilities are likely to decline, with cognitive dysfunction improving after initiation of dialysis and even being generally reversible after successful renal transplantation. However, little is known about cognitive changes particularly regarding different treatment modalities. To gain further insight into this, we focused on a one-year course of cognitive functions, comparing peritoneal to hemodialysis patients. Within the CORETH-project, two validated neurocognitive tests, assessing executive functioning (Trail Making Test-B) and attention (d2-Revision-Test) and the self-reported Kidney Disease Quality of Life Short Form Cognitive Function-subscale, were administered to 271 patients at baseline and after one year. Subsamples were matched by propensity score, adjusting for age, comorbidity, education, and employment status for 96 hemodialysis and 101 peritoneal dialysis patients. The effects of time and treatment modality were investigated, controlling for well-known confounders. Both tests revealed improvement over one year. Peritoneal dialysis was associated with better outcomes than hemodialysis at baseline and follow-up, but comparability between groups may be limited. The opposite pattern applied to self-reporting. Hemodialysis patients had to be excluded from cognitive testing more often than peritoneal dialysis patients. As such, the number of exclusions may have biased the findings, limiting generalizability. Thus, our findings suggest an improvement of cognitive functioning and support previous indications for peritoneal dialysis being associated with better cognitive functions during a one-year course than hemodialysis.