The influence of 4 months of physical training (PT) and detraining on body composition, and risk factors for coronary artery disease (CAD) and noninsulin-dependent diabetes mellitus (NIDDM) was examined. The subjects were 81 obese 7-11-year-olds. At baseline, visceral adipose tissue (VAT) was the main adiposity variable cross-sectionally associated with unfavorable levels of the lipid-lipoprotein risk factors, while fat mass was more highly correlated with insulin, systolic blood pressure, and leptin. Adiposity measures were associated with unfavorable concentrations of clotting-fibrinolysis factors. Subjects were randomly assigned to engage in PT for the first or second 4-month period of the study; for most variables, tests were done at 0, 4, and 8-month time points. The PT program was offered 5 days a week for 40 min/session. For the 73 children who completed 4 months of PT, attendance was 80% and heart rate during the sessions was 157 bpm. PT had a favorable influence on percent fat, VAT, subcutaneous abdominal adipose tissue, insulin, triacylglycerol, and cardiac parasympathetic activity. Detraining generally led to unfavorable changes in percent fat and associated risk factors. Leptin decreased during periods of PT and increased following cessation of PT. No significant changes due to PT were found for diet, hemodynamic, left ventricular, or most lipid parameters. Thus, 4 months of controlled PT, without dietary intervention, had a favorable impact on body composition and some obesity-associated CAD/NIDDM risk factors. Am. J. Hum. Biol. 11:237-247, 1999. Copyright 1999 Wiley-Liss, Inc.