Infection induces changes in the serum lipoprotein pattern in man. In this report the concentration of cholesterol and triglycerides in the major serum lipoprotein classes were followed in 9 patients with severe bacterial infections. Blood samples for lipoprotein analysis were obtained in the fasting state the first 4 days after admission to the hospital and after 2-3 weeks and 2 months. The serum lipoprotein concentrations of the patients were compared with those from a group of healthy subjects. The total serum cholesterol concentration was lowered during the acute stage of the disease and remained low the first days in hospital. The very low (VLDL) density lipoprotein cholesterol level in serum was mainly within the normal range. The low (LDL) density lipoprotein cholesterol values in serum were low during the first 4 days in hospital. The high (HDL) density lipoprotein cholesterol concentration values were extremely decreased on the first day in hospital and had a tendency to further reduction from day 1 to day 4. Both the LDL and the HDL serum cholesterol were normalized after recovery.