This study aimed to investigate the association of the anthropometric, clinical variables and maximal oxygen uptake (V<sc>o</sc>_2max) with diabetes-specific health-related quality of life (HRQoL) in youths with type 1 diabetes mellitus (T1DM) and to find the predictors of HRQoL and blood glucose control. A total of 239 youths with diabetes (124 boys and 115 girls) were recruited from diabetes-based summer camps. HRQoL assessment was carried out with the Pediatric Quality of Life Inventory 3.0 Diabetes Module (Information Resources Centre, Mapi Research Trust, France); V<sc>o</sc>_2max was evaluated by conducting the 20-m shuttle run test. Higher V<sc>o</sc>_2max and the insulin pump therapy were significant predictors of the HRQoL in the multiple regression analysis; other clinical and anthropometric variables had no effect. The better blood glucose control was explained only by the higher V<sc>o</sc>_2max. The good cardiorespiratory fitness (expressed by V<sc>o</sc>_2max) has clinical and QoL benefits for paediatric patients with T1DM.