Plasma lipid and lipoprotein composition was investigated in acute renal failure (ARF). Forty-seven percent of the patients showed hypertriglyceridemia, whereas serum cholesterol was slightly reduced. Triglyceride (TG) content, predominantly of LDL and to a lesser degree of VLDL, was elevated. Cholesterol concentrations of HDL and LDL fractions were markedly reduced. HDL-TG and VLDL-cholesterol were in the normal range. Forty percent of the patients had a type IV hyperlipoproteinemia. Post-heparin lipolytic activity (PHLA) was reduced owing to an inhibition of hepatic-TG-lipase (HTGL) activity, whereas protamine-inactivated LPL was in the normal range. Fractional elimination (K2) of parenterally administered fat emulsions, determined by an intravenous fat tolerance test (IVFTT) was reduced to 2.44% min, about half of normal. The fat elimination rate increased but did not normalize during parenteral nutrition with amino acids and glucose, suggesting enhanced lipid deposition. Alterations of lipid metabolism develop early in ARF (by 4 days) and, in general, are not influenced by residual renal function, urinary output, or duration of renal failure.