Objective-To investigate whether the risks of mortality from brain cancer are related to occupational exposure to magnetic fields. Methods-A total of 112 cases of primary brain cancer were identified from a cohort of 84 018 male and female employees of the (then) Central Electricity Generating Board and its privatised successor companies. Individual cumulative occupational exposures to magnetic fields were estimated by linking available computerised job history data with magnetic field measurements collected over 675 person-workshifts. Estimated exposure histories of the case workers were compared with those of 654 control workers drawn from the cohort (nested case-control study), by means of conditional logistic regression. Results-For exposure assessments based on arithmetic means, the risk of mortality from brain cancer for subjects with an estimated cumulative exposure to magnetic fields of 5'4-13-4 uT.y v subjects with lower exposures (0.0-5.3 pT.y) was 1 04 (95% confidence interval (95% CI) 0-60 to 1-80). The corresponding relative risk in subjects with higher exposures (> 13 5 pT.y) was 095 (95% CI 0 54 to