Aims The aims of the study were to explore the effects of long-term endurance exercise on atrial and ventricular size and function in adolescents and to examine whether these changes are related to maximal oxygen uptake (VO2max). Methods and results Twenty-seven long-term endurance-trained adolescents aged 13-19 years were individually matched by age and gender with 27 controls. All participants, 22 girls and 32 boys, underwent an echocardiographic examination at rest, including standard and colour tissue Doppler investigation. VO2max was assessed during treadmill exercise. All heart dimensions indexed for body size were larger in the physically active group compared with controls: left ventricular end-diastolic volume 60 vs. 50 mL/m(2) (P amp;lt; 0.001), left atrial volume 27 vs. 19 mL/m(2) (P amp;lt; 0.001), and right ventricular (RV) and right atrial area 15 vs. 13 and 9 vs. 7 cm(2)/m(2), respectively (P amp;lt; 0.001 for both). There were strong associations between the size of the cardiac chambers and VO2max. Further, we found improved systolic function in the active group compared with controls: left ventricular ejection fraction 61 vs. 59% (P = 0.036), tricuspid annular plane systolic excursion 12 vs. 10 mm/m(2) (P = 0.008), and RV early peak systolic velocity s 11 vs. 10 cm/s (P = 0.031). Conclusion Cardiac remodelling to long-term endurance exercise in adolescents is manifested by an increase in atrial as well as ventricular dimensions. The physically active group also demonstrated functional remodelling with an increase in TAPSE and systolic RVwall velocity. These findings have practical implications when assessing cardiac enlargement and function in physically active youngsters.