Objective: To investigate the association between the duration of weekly leisure-time sports activity and all-cause mortality. Methods: As part of the prospective Copenhagen City Heart Study, 8697 healthy adults completed a comprehensive questionnaire about leisure-time sports activities. Duration (minutes per week) of leisure-time sports activities was recorded for tennis, badminton, soccer, handball, cycling, swimming, jogging, calisthenics, health club activities, weightlifting, and other sports. The primary end point was all-cause mortality, and the median follow-up was 25.6 years. The association between duration of leisure-time sports activities and all-cause mortality was studied using multivariable Cox proportional hazards regression analysis. Results: Compared with the reference group of 2.6 to 4.5 hours of weekly leisure-time sports activities, we found an increased risk for all-cause mortality for those with 0 hours (hazard ratio [HR], 1.51; 95% CI, 1.29 to 1.76), for those with 0.1 to 2.5 hours (HR, 1.24; 95% CI, 1.05 to 1.46), and for those with more than 10 hours (HR, 1.18; 95% CI, 1.00 to 1.39) of weekly leisure-time sports activities. These relationships were generally consistent with additional adjustments for potential confounders among subgroups of age, sex, education, smoking, alcohol intake, and body mass index, when the first 5 years of follow-up were excluded, and for cardiovascular disease mortality. Conclusion: We observed a U-shaped association between weekly duration of leisure sports activities and cardiovascular and all-cause mortality, with lowest risk for those participating in 2.6 to 4.5 weekly hours, being consistent across subgroups. Participation in sport activities should be promoted, but the potential risk of very high weekly hours of sport participation should be considered for inclusion in guidelines and recommendations.