This study was designed to find out about the body composition of ordinary Japanese adults, and to clarify how gender and age have effects on it. The subjects were volunteers living in the northern, central and western parts of Japan. They consisted of 154 males and 142 females aged 20 to 59 years. Their body height and weight did not differ by more than 1 S. D. from the mean of the given age category cited in the Japanese Ministry of Education Annual Report. Body composition was estimated from densitometry using an underwater weighing method and pulmonary residual volume measurement. Within each age group, there were significant differences in body height, weight, body density, percentage body fat, lean body weight, fat per 1 m of body height and lean body weight per 1 m of body height, but there was no significant difference in fat between males and females. Percentage body fat and fat increased significantly with age in both genders. The rate of increases of fat were calculated to be 14.2% for males and 10.7% for females per decade. Lean body weight decreased significantly with age in males, decreasing at a rate of 3.3%. However, the lean body weight of females stayed constant over the given ages of this study. These results show that the difference in body weight between males and females is caused by lean body mass, not by fat.