To determine the effect of obesity on the size distribution of fat cell populations in human adipose tissue, omental fat tissue biopsies were obtained from lean, moderately obese, and massively obese patients. The size distributions of adipocytes from lean and obese fat tissues examined by the scanning electron microscopic method were bimodal, consisting of populations of very small fat cells and mature fat cells, in contrast to collagenase-derived isolated cells that showed only the large mature fat cells. The very small fat cell population represented 21 to 26% of the total fat cell number in the lean and in both obese groups. In contrast, preparations of human fat cells isolated by the collagenase method systematically excluded the very small fat cells. In massive obesity, both cell populations participated in the hyperplastic growth but only the larger mature fat cells increased in size, implying that these two cell populations differ in their physiological role.