OBJECTIVE To measure cardiac function before and after 3 months of aerobic interval training in obese adolescents and to compare the findings with those in lean counterparts. METHODS Exercise intervention study. Cardiac function was assessed by echocardiography and maximal oxygen uptake by ergospirometry. METHODS The obese adolescents were referred from general practice to the St Olav University Hospital, Trondheim, Norway, and the control group was recruited from 2 schools. METHODS Ten obese adolescents (mean [SD] age, 14.8 [1.2] years; mean [SD] body mass index {BMI; calculated as weight in kilograms divided by height in meters squared}, 33.5 [4.3]) and 10 lean counterparts (mean [SD] age, 14.9 [1.3] years; mean [SD] BMI, 20.4 [3.0]) participated. Intervention Aerobic interval training (4 x 4 minutes at 90% of maximal heart rate, 40 minutes of training in total) was performed twice per week for 13 weeks among the obese adolescents, whereas the lean counterparts only performed the tests. METHODS Left ventricular end-diastolic volume, stroke volume, and maximal oxygen uptake. RESULTS Maximal oxygen uptake was 41.4% lower among the obese adolescents compared with the lean counterparts, but the maximal oxygen uptake increased by 8.6% (P = .008) after intervention. Obese adolescents initially had 7.8% and 14.5% lower left ventricular end-diastolic and stroke volumes, 21.3% reduced global strain rate and 16.3% global strain, reduced mitral annulus excursion and systolic/diastolic tissue velocity, longer isovolumic relaxation time, and longer deceleration time compared with the lean counterparts. No group difference was observed after the intervention. Aerobic interval training increased the ejection fraction but was lower compared with the lean counterparts. Aerobic interval training reduced fat content by 2.0% (P = .005) among the obese adolescents. CONCLUSIONS Aerobic interval training almost restored an impaired systolic and diastolic cardiac function among obese adolescents when compared with lean counterparts. These results may have implications for future treatment programs for obese adolescents.