Background Cardiac resynchronization therapy (CRT) improves quality of life (QoL) when measured 3 to 6 months after implantation, but whether these effects are sustained is unknown. The CArdiac Resynchronisation-Heart Failure study is the only long-term randomized trial of CRT with repeated measures of QoL. Methods Quality of life was measured at baseline and 3 months using generic European Quality of Life-5 Dimensions and disease-specific (Minnesota Living with Heart Failure) questionnaires and at 18 months and study-end using the latter instrument. Median follow-up was 29.6 (interquartile range 23.6-34.6) months. Results At baseline, patients had a substantially impaired QoL (mean European Quality of Life-5 Dimensions score 0.60, 95% confidence interval CI 0.58-0.62) compared to an age-matched general population (0.78, 95% CI 0.76-0.80). Quality of life improved to a greater extent in patients assigned to CRT at each time point ( P < .0001). By 18 months, the mean difference in disease-specific QoL score was 10.7 (95% CI 7.6-13.8) in favor of CRT, mostly due to improved physical functioning. Differences were sustained thereafter. Quality-adjusted life-years at 18 months increased by 0.13 (95% CI 0.07-0.182) and by 0.23 (95% CI 0.13-0.33) at study-end (both P < .0001). Little heterogeneity of effect across subgroups was observed. Conclusion Cardiac resynchronization therapy improves long-term QoL and survival in patients with moderate to severe heart failure. The effects appear sustained, and therefore, the gain in quality-adjusted life years with CRT should be even greater during longer term follow-up.