BACKGROUND: Acute kidney injury (AKI) is a complication after major cardiac surgery that is associated with higher rates of morbidity and mortality. MicroRNA-21 (miR-21) has been described as an early biomarker for AKI. We investigated whether miR-21 is predictive of AKI and long-term mortality after cardiac surgery. METHODS: Consecutive patients (n = 115) undergoing major cardiac surgery were included. Serum creatinine was measured prior to, 4 h after, and 1, 4 and 7 days after extracorporeal circulation. Diagnosis of post-operative AKI was made in accordance with the international Kidney Disease: Improving Global Outcomes definition of AKI. Serum cystatin C and miR-21 were measured prior to and 4 h after surgery. miR-21 was determined by quantitative RT-PCR and was normalized to miRNA-39 from Caenorhabditis elegans. The median follow-up time was 2.9 years. RESULTS: AKI occurred in 36.5% (n = 42) of all patients. Baseline miR-21 was significantly lower in patients developing cardiac surgery-associated AKI (CSA-AKI) than in patients without CSA-AKI [0.27 (interquartile range, IQR, 0.14-0.30) versus 0.44 (IQR 0.25-0.75); P < 0.01]. Baseline miR-21 predicted CSA-AKI Stage 2/3 with an area under the curve of 0.701 [95% confidence interval (CI) 0.59-0.82; P = 0.007]. Baseline miR-21 <0.31 showed a hazard ratio of 3.11 (95% CI: 1.33-11.26) for CSA-AKI Stage 2/3. Patients with AKI Stage 2/3 had a significantly higher mortality (50 versus 10%; P = 0.0001) and dialysis rate (27 versus 11%; P = 0.038) within the 2.9-year follow-up. CONCLUSIONS: Our results indicate that miR-21 has the potential to identify patients at higher risk for CSA-AKI. This predictive value might be helpful in pre-procedural risk assessment and peri-procedural diagnosis and treatment.