The most malignant form of all brain tumors is the supratentorial astrocytoma. Little is known about its etiology, but exogenous factors have been blamed. In this case-control study, 78 astrocytoma patients have been compared with 197 clinical and 92 population controls. An extensive questionnaire was used to gather information about occupational and residential environment exposure. Inquiries concerning groups of or individual chemicals elicited low rates of affirmative response, with negligible differences between cases and controls. However, the questions "working at an airfield" and "living near a petrochemical plant" indicated elevated risks in comparison with both control groups; so too did "living near a municipal sewage treatment plant." These results focus attention on exposure to organic compounds and should be considered together with similar findings in current research. No other occupation, branch of industry, or vicinity questions showed differences between cases and controls, with the exception of "living in the neighborhood of a paper mill or a saw mill," which gave moderately increased relative risks. A separate report gives the results from the nonoccupational part of the study.