3-Hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase, the rate-limiting enzyme of cholesterol biosynthesis, was assayed in human brain tumors. Enzyme activity was low or absent in most primary brain tumors, but activity was high in two metastatic tumors. Differences in enzyme activity were consistent with differences in incorporation of [1-14C]acetate into tumor sterols, but were not correlated with tumor sterol content. In nonneural tumors, HMG-CoA reductase activity was highest in adenocarcinomas of colon and stomach. Compounds that suppress HMG-CoA reductase activity arrest the growth of neoplastic cells in vitro. The data suggest that certain classes of human tumors might be susceptible to a chemotherapeutic approach based on inhibition of this enzyme.