Abstract The relationships between cardiac dimensions and physical activity and submaximal working capacity were examined in 198 boys and 154 girls, aged 918years, who were participants in the first phase of the Qubec Family Study. The sample was divided into three age groups, 912years, 1315years, and 1618years. Indicators of physical activity included estimated daily energy expenditure (EE) and time spent in moderate-to-vigorous physical activity (median metabolic equivalents of energy expenditure above resting metabolic rate 4.8). Submaximal physical working capacity (PWC150) was determined using a submaximal exercise test on a Monark cycle ergometer. Echocardiographically determined dimensions included posterior wall thickness, septal wall thickness, and left ventricular mass (LVM). The analyses were based on partial correlation and analysis of covariance, controlling for age and body surface area. Relationships between EE/physical activity variables and cardiac dimensions were low and, at best, moderate (r0.45). With subjects grouped into tertiles by indicators of physical activity, LVM was significantly different only among 16- to 18-year-old girls (157g vs 134g in the highest and lowest quartiles, respectively; P0.05). Correlations between cardiac dimensions and PWC150 were also low (r0.30), with few significant relationships. In general, cardiac dimensions were not related to habitual physical activity and PWC150 in young subjects aged 918years. However, significant correlations were positive, as expected. LVM may be related to submaximal power output in boys since it accounts for 3% of the variance, after adjusting for age and BSA.