Previous studies on the etiology of obesity have revealed that human adipocytes have the ability to revert or dedifferentiate in culture to a morphology and replicative capacity similar to that of adipocyte precursors. To characterize some of the events of this process, we isolated adipocytes from the greater omentum of 61 morbidly obese and ten normal weight individuals with collagenase, and cultured them for 0, 4, and 7 days. In both lean and obese patients, sn-glycerol-3-phosphate dehydrogenase specific activity decreased significantly after days 4 and 7 compared to day 0. Dedifferentiation was also monitored by phase-contrast microscopy, which revealed that adipocytes from the lean had lost appreciable lipid and had assumed an elongated contour more rapidly than those from the obese. Reversion was also corroborated by reverse transcription-polymerase chain reaction, which indicated a decrease in the expression of sn-glycerol-3-phosphate dehydrogenase mRNA, and an increase in actin and glyceraidehyde-3-phosphate dehydrogenase mRNA over the 7 days. Thus, this work has described some biochemical and molecular genetic characteristics of dedifferentiation. The relative resistance of adipocytes from morbidly obese patients to revert in culture may reflect the inordinately high propensity of fat cells in massively obese persons to preserve the differentiated, triacylglycerol-overfilled state.