The hyperlipidemia accompanying infection has been attributed to production of tumor necrosis factor. This cytokine inhibits adipose tissue lipoprotein lipase, which could decrease clearance of lipoproteins. Infections also increase hepatic lipogenesis. We now have demonstrated that tumor necrosis factor-alpha stimulates lipid synthesis in vivo. 2 h after administration of tumor necrosis factor (25 micrograms/200 g), plasma triglycerides increase 2.2-fold and remain elevated for 17 h. Plasma cholesterol also increases, but this effect appears after 7 h. Tumor necrosis factor rapidly stimulates incorporation of tritiated water into fatty acids in the liver (1-2 h), which persists for 17 h. Also, tumor necrosis factor stimulates hepatic sterol synthesis. Of note, tumor necrosis factor treatment does not stimulate lipid synthesis in other tissues, including adipose tissue. Labeled fatty acids rapidly increase in the plasma, raising the possibility that stimulation of hepatic lipogenesis by tumor necrosis factor contributes to the hyperlipidemia of infection.