Plasma lipids and lipoprotein cholesterol concentrations were determined before and at 3 months and 1 year after partial ileal bypass surgery in 28 male survivors of first myocardial infarction (eight normolipidemic subjects and eight type II-A, two type II-B, eight type IV and two type V hyperlipoproteinemic subjects). All subjects had marked reductions in plasma total cholesterol (average 45% and 33% in the type V subjects and 37% and 31% in the other 26 subjects at 3 months and 1 year after surgery). Except for the two type V subjects, all had even more marked reductions in low-density lipoprotein (LDL)-cholesterol than in the total plasma cholesterol, averaging 51% at 3 months and 49% at 1 year after surgery. There were no significant changes in high-density lipoprotein (HDL)-cholesterol levels. The hypertriglyceridemic subjects had marked reductions in plasma triglycerides and very low density lipoprotein-cholesterol, whereas the normotriglyceridemic subjets (normals and II-A) had slight increases in these two measurements after surgery. Partial ileal bypass tends to normalize elevated plasma lipid and lipoprotein levels and results in a maximal lowering in LDL-cholesterol concentration without altering the HDL-cholesterol level.