The aims of this work were to establish whether, in an obese population, a group characterized by a distinctly high fact cell number could be observed and whether the number of body fact cells was affected by slimming. In the obese group there were 75 obese females (kg/m2 greater than 27.0) and 19 obese males (kg/m2 greater than 27.5); the lean group contained 37 females and 11 males. Body fat was estimated either from skinfold thickness or from body potassium measurements. Fat cell size was measured by a well-established microscopic method. No clear distinction could be made between types of obese patients on the basis of their estimated total number of measurable body fact cells. This number was weakly related to the degree of obesity (r = 0.342; P less than 0.001). Weight reduction caused no appreciable change in fat cell number. We conclude that the whole concept of adipocyte hypercellularity as a causal factor in obesity has been overemphasized.