Longer dialysis session length (treatment time, TT) has been associated with better survival among hemodialysis (HD) patients. The impact of TT on clinical markers that may contribute to this survival advantage is not well known.METHODS: Using data from the international Dialysis Outcomes and Practice Patterns Study, we assessed the association of TT with clinical outcomes using both standard regression analyses and instrumental variable approaches. The study included 37 414 patients on in-center HD three times per week with prescribed TT from 120 to 420 min.RESULTS: Facility mean TT ranged from 214 min in the USA to 256 min in Australia-New Zealand. Accounting for country effects, mortality risk was lower for patients with longer TT hazard ratio for every 30 min: all-cause mortality: 0.94 [95 confidence interval (CI): 0.92-0.97], cardiovascular mortality: 0.95 (95 CI: 0.91-0.98) and sudden death: 0.93 (95 CI: 0.88-0.98) . Patients with longer TT had lower pre- and post-dialysis systolic blood pressure, greater intradialytic weight loss, higher hemoglobin (for the same erythropoietin dose), serum albumin and potassium and lower serum phosphorus and white blood cell counts. Similar associations were found using the instrumental variable approach, although the positive associations of TT with weight loss and potassium were lost.CONCLUSIONS: Favorable levels of a variety of clinical markers may contribute to the better survival of patients receiving longer TT. These findings support longer TT prescription in the setting of in-center, three times per week HD.