This study had two objectives. First, we examined whether the influence of diet combined with either aerobic (DA) (n = 10) or resistance (DR) (n = 10) exercise has effects on insulin and glucose levels that are different in obese men. Second, we tried to determine whether the combination of diet and exercise is associated with improvements in insulin and glucose levels that are greater than those associated with diet alone (DO) (n = 9). Insulin and glucose levels were measured after an overnight fast and a 75-g oral glucose challenge (OGTT). Visceral adipose tissue (AT), subcutaneous AT, and skeletal muscle were measured by magnetic resonance imaging (MRI) before and after treatment (16 weeks). Reductions in weight (12.4 +/- 3.8 kg) and in visceral (37 +/- 15.1%) and subcutaneous AT (24.3 +/- 8.6%) were not different between treatments (P > 0.05). Skeletal muscle was maintained in the DA and DR groups but was reduced (7.3 +/- 2.8%) in the DO group (P < 0.05). Independent of treatment, fasting glucose and OGTT glucose did not change (P > 0.05). However, fasting insulin, OGTT insulin, and the insulin-to-glucose ratio decreased within all treatments (P < 0.05). Reductions in the OGTT insulin area under the curve were greater (P < 0.05) within the DA (52 +/- 12%) and DR (42 +/- 17%) treatments than in the DO (20 +/- 15%) treatment. Collapsed across group, reductions in visceral AT were related to reductions in fasting and OGTT glucose (P < 0.05), whereas reductions in abdominal subcutaneous AT correlated with reductions in fasting insulin (P < 0.05). Weight loss induced by diet and aerobic or resistance exercise has similar positive effects on lowering fasting and OGTT insulin values that are greater than those with diet alone. Because changes in glucose and insulin were related to reductions in visceral and abdominal subcutaneous AT, we conclude that reduction in abdominal obesity consequent to diet and exercise-induced weight loss is important for attaining improvements in plasma insulin levels, observations that strengthen the concept that abdominal obesity has an important role in mediating insulin resistance.