We conducted a case-control study to test the hypothesis that exposure to magnetic fields of the type generated by high-voltage power lines increases the incidence of leukemia and central nervous system tumors in adults. The study was based on people who, between 1960 and 1985, had lived on a property in Sweden located within 300 meters of 220 or 400 kilovolt power lines. We identified a total of 325 leukemia cases and 223 cases of central nervous system tumor. Two matched controls per case were selected at random. We assessed exposure by spot measurements and by calculations of the magnetic fields generated by the power lines. For calculated magnetic field levels of 0.2 microT or more closest in time to diagnosis, we found an elevated relative risk (RR) for acute myeloid leukemia [RR = 1.7; 95% confidence interval (CI) = 0.8-3.5] and chronic myeloid leukemia [RR = 1.7; 95% CI = 0.7-3.8]. Using cumulative exposure for the 15 years preceding diagnosis, we found relative risk estimates for acute and chronic myeloid leukemia of 2.3 (95% CI = 1.0-4.6) and 2.1 (95% CI = 0.9-4.7), respectively, for the highest exposure category. For chronic lymphatic leukemia and for central nervous system tumors, relative risk estimates were close to or below unity.