Risk factors of stroke/thromboembolism (TE) and major bleeding, and incidence of these events in specific age categories in warfarin-treated patients with mechanical heart valves (MHV) are uncertain. Our objective was to calculate event rates in specific age categories and identify risk factors for adverse events. Methods and results We identified 4,810 treatment periods with MHV between January 2006 and December 2011 in the Auricula and Swedish Web system for Enhancement and Development of Evidence-based care in Heart disease Evaluated According to Recommended Therapies registries. There were 3,751 treatment periods with aortic valve replacements (AVR) and 866 with mitral valve replacements (MVR). Median follow-up time was 4.5 years (IQR, 1.5-6.0). Time in therapeutic range with warfarin for patients with AVR was 74.2% for international normalized ratio of 2.0 to 3.0, with 72% of the patients having this target range. Rate of stroke/TE for AVR and MVR was 1.3 and 1.6 per 100 patient years, respectively (P = .20). The rate of first major bleeding was 2.6 and 3.9 per 100 patient years with AVR and MVR, respectively (P < .001). By multivariate analysis for AVR, age (hazard ratio [HR], 1.02; 95% confidence interval [CI], 1.01-1.03 per year) and previous stroke (HR, 2.4; 95% CI, 1.7-3.5) emerged as independent risk factors for stroke/TE. Heart failure (HR, 0.9; 95% CI, 0.6-1.4) and atrial fibrillation (HR, 1.0; 95% CI, 0.7-1.4) were not associated to stroke/TE. For major bleeding events, age (HR, 1.02; 95% CI, 1.01-1.03 per year) and previous major bleeding (HR, 2.5; 95% CI, 1.9-3.3) emerged as independent risk factors for AVR. Conclusions In a nationwide cohort study with MHV and high time in therapeutic range, heart failure and atrial fibrillation did not appear as risk factors of stroke/TE.