Ten obese patients were subjected to physical training, which resulted in an increased maximal oxygen consumption and an increased isometric muscle strength. Body weight increased, due primarily to an increase in body fat, but also, in some cases, to an increase in body cell mass, determined by isotope dilution techniques. Fat cell diameter was unchanged. Peroral glucose tolerance test with plasma radioimmuochemically determined insulin in these patients showed no changes in blood glucose values after training but a marked decrease in insulin values. This was interpreted to be due to an increased insulin sensitivity of tissues. Since the body fat mass was not decreased it was not considered likely that the increased insulin sensitivity was due to adipose tissue factors. This augmentation of insulin sensitivity was furthermore not related to the increase in body cell mass and therefore probably not to an increase in muscle mass. It was concluded that muscle probably is an important determinant for insulin sensitivity in obesity.