Abstract.Fourteen grossly obese patients were studied before and 2, 6, 12 and 24 months after jejuno‐ileal bypass. The major weight loss occurred during the first 6 months while there was no significant change of the mean body weight during the last 12 months of the study. During the initial period of pronounced weight loss there were significant reductions of all lipoprotein classes, e.g. at 2 months the very low density lipoprotein triglycerides had decreased by 41% (P<0.01), the low density lipoprotein cholesterol by 42% (P<0.001), the high density lipoprotein cholesterol by 27% (P<0.001) and the serum apolipoprotein B, A‐I and A‐II concentrations by 30% (P<0.001), 17% (P<0.001) and 27% (P<0.01) respectively. The fractional removal rate (K2) at the intravenous fat tolerance test had increased by 38% (P<0.01), indicating an increased removal capacity of triglyceride‐rich lipoproteins. At 24 months the very low density lipoprotein triglycerides, the low density lipoprotein cholesterol and the apolipoprotein B concentration remained reduced by 28% (P<0.05), 48% (P<0.001) and 35% (P<0.001) respectively. The intravenous fat tolerance testK2remained significantly increased by 44% (P<0.05). However, the high density lipoprotein cholesterol and the apolipoprotein A‐I and A‐II concentrations had successively increased again to pre‐operative levels. The low density lipoprotein cholesterol/high density lipoprotein cholesterol ratio was decreased by 44% (P<0.001) and the apolipoprotein A‐I/apolipoprotein B ratio had increased by 74% (P<0.001), both changes presumably beneficial in regard of atherogenicity. The apolipoprotein A‐I/high density lipoprotein cholesterol and apolipoprotein B/low density lipoprotein cholesterol ratios had increased by 23% (P<0.001) and 28% (P<0.001) respectively. The data indicate that the reduction of high density lipoprotein may be a transient phenomenon during the initial period of weight loss but that definite changes occur in the high density lipoprotein and low density lipoprotein composition which are apparent also 2 ye