Although many investigations have been conducted to determine the effects of exercise and/or diet prescription, seasonal variations in weight loss have not been thoroughly investigated in Japan. The present investigation was undertaken to determine seasonal variations in body composition and anthropometric characteristics during a weight-loss program. One hundred and twenty-seven women [mean (SD) age 44.3 (11.7) years] participated in this study. The subjects were categorized into four groups based on the season in which the weight-loss program was begun: subjects who joined our exercise program in winter (group WE), subjects who joined in summer (group SE), and two control groups, one for winter and one for summer (group WC and group SC, respectively). The subjects of groups WE and SE completed a 3-month exercise and food-restriction program. Analyses of covariance revealed that the subjects of group WE seemed to have attained more desirable changes. This group showed a decrease in body mass (-1.9 (1.5) kg) and percent body fat (-4.6 (4.8)%), while an increase in fat-free mass (1.6 (3.2) kg) during the experimental period. Our data suggest that the better season for desirable weight loss in Japan is winter, compared to summer. These alterations may be attributable, at least in part, to the fact that Japanese people are generally acclimatized to a cold climate in this season. The mechanisms involved in our speculation have not been well validated, but it seems clear that a significant difference in dietary intake might have affected the results of this study.