Body composition and total number of fat cells were investigated in 31 randomly selected women 52 yr of age and in 13 young women (mean age 22 yr) whose body weights were within plus or minus 10% of the ideal weight. Two of the 52-year-old women were obviously obese and excluded. Regional determinations of adipose tissue thickness, fat cell size, and number were also determined. Middle-aged women had more body fat (BF) but a lower body cell mass (BCM) than the younger group. The increased BF in the middle-aged women was exclusively explained by larger fat cells, since the younger women had a significantly higher total number of fat cells. This increase was also found when differences in height, body weight, and body fat were matched out. Local fat cell number was also increased in the younger group. Local fat cell size was increased in all regions investigated in the middle-aged women, but the increase was particularly pronounced in the abdominal region. The highest degrees of correlation between fat cell sizes of different regions were found between the epigastric and hypogastric regions and between femoral and gluteal regions. Furthermore, the plasma insulin levels correlated with the fat cell sizes of the abdominal region but not with those of the femoral or gluteal regions. In conclusion, the data might indicate that the fat cells of the abdominal region are more sensitive to nutritional and/or hormonal factors than those of other regions. This may in turn indicate the existence of different fat cell populations.