The effect of an intensive physical training program on body weight and skinfold thickness was compared in subjects who had initially small (group S), medium (group M) and large (group L) skinfold thickness representing mostly thin, normal and obese recruits, respectively. Weight and skinfold thickness at three sites (triceps, subscapula and abdomen) were measured at the beginning and end of a 6 week intensive military training program. Subjects were on an ad-lib diet. Seventy-one percent of the subjects in group S increased their body weight and 60% increased the sum of their skinfold thickness (SSFT) at the three sites during the training period. These increases were significant at P less than 0.001 and P less than 0.01 for weight and SSFT respectively. In groups M and L the trends for changes in SSFT were in the opposite direction, with 68% of the former and 98% of the latter showing decreases in SSFT. The mean decreases were significant at P less than 0.02 and P less than 0.001 for groups M and L respectively. While the subjects in group L lost weight significantly (P less than 0.001), those in group M reduced their weight only slightly. Our data indicate that changes in body fat, indicated by changes in SSFT, during several weeks of physical activity are not unidirectional and are related to the initial skinfold thickness.