The rate of incorporation of glycerolcarbon into triglyceride (TG) in liver slices obtained from 17 severely obese normolipoproteinemic patients during jejunoileal bypass was significantly elevated compared to normal controls. The obese patients had an almost sevenfold increase in hepatic TG content as well. None of the patients had manifest diabetes, though increased basal levels of insulin and impaired glucose tolerance during oral glucose tolerance tests (OGTT) were common. There were statistically significant positive Spearman rank correlations between plasma insulin levels and TG synthesis (r = 0.57; p < 0.01), TG content (r = 0.54; p < 0.01), and serum fatty acids (r = 0.52; p < 0.05). The basal insulin levels were also positively correlated to the incorporation rate of fructose-carbon into fatty acids (r = 0.47; p < 0.05). The sum of insulin values during OGTT showed a trend toward positive correlation with the hepatic cholesterol content (r = 0.34; p < 0.10). The hepatic protein content as well as the rate of leucinecarbon incorporation into proteins did not differ from controls. However, there was a trend toward a negative correlation between protein synthesis and basal plasma insulin levels (r = −0.33; p < 0.10). It is suggested that liver steatosis in severely obese patients is due to significantly increased hepatic lipid synthesis in the face of elevated levels of serum fatty acids. The possible importance of plasma insulin is also emphasized.