Background— Although concentrating adult congenital heart disease services at high-volume centers has been widely advocated, the potential beneficial effects of competition and patient choice have received relatively little attention. We aimed to assess the degree of patient choice and competition between adult congenital heart disease units and to investigate whether competition indices correlate with clinical quality or research output. Methods and Results— Competition between the 10 major adult congenital heart disease units in England was evaluated based on the Herfindahl–Hirschman Index, representing the sum of squared market shares of individual units. In addition, to account for geography and feasible access, we calculated spatial indices of competition based on travel time by road. These indices were correlated with 30-day mortality postpulmonary valve replacement in adult patients (as obtained from the National Central Cardiac Audit Database) and the aggregate research impact factors of individual centers. On a national level, a high level of competition without obvious dominant players was found (Herfindahl–Hirschman Index between 0.107 and 0.013). When accounting for geography, however, important disparities in patient choice and competition faced by individual centers emerged. The degree of local competition was correlated significantly with clinical outcomes and research output. In contrast, no association between center volume and outcome could be established. Conclusions— Beyond the usual focus on concentrating services at high-volume centers, the potentially beneficial effects of competition should not be ignored. Therefore, policymakers should consider fostering a competitive environment for adult congenital heart disease centers or at least avoiding creating government-granted monopolies in the field.