Abnormalities in serum lipids, including hypertriglyceridemia, are common during infectious disorders. However, the lipoprotein pattern during infections, particularly in children, has been investigated to only a limited extent. We have monitored alterations in serum lipoproteins in eight children with a severe bacterial infection (meningitis) by a quantitating method measuring cholesterol and triglycerides in each major class. The levels of triglycerides in serum and in low-density lipoproteins were markedly elevated during the infection, whereas the amount of cholesterol in high-density lipoproteins was decreased. The cholesterol to triglyceride ratio was decreased in low-, as well as in high-density lipoproteins. These lipoprotein abnormalities may, at least in part, be explained by a depressed lipolytic activity of lipoprotein lipase, the key enzyme for removal of triglycerides in man. Serum triglycerides and the levels of cholesterol in high-density lipoproteins, as well as the ratio between these parameters, may be used as indicators of inflammatory activity.