Gamification has been increasingly employed in health-related apps in recent years. However, the effect of gamification design on the success of health and fitness apps remains unknown and has not been investigated before. This study attempts to identify what gamification elements are frequently used in the design of health and fitness apps and to empirically quantify their effects on app downloads and user ratings of these apps. We construct a rich dataset that includes information about the daily downloads, ratings and gamification design elements of 2,462 health and fitness apps on the Apple App Store. Our sample contains 924 paid apps and 1,538 free apps. This study contributes to both the gamification and mobile app literatures and provides important implications for app developers who intend to adopt gamification in mobile app design.