Several studies have raised the possibility that exposure to electrical and/or magnetic fields may be injurious to health in particular by the promotion or initiation of cancer. To investigate whether the electricity transmission system presents a long term hazard to public health, the mortality of nearly 8,000 persons, identified as living in the vicinity of electrical transmission facilities at the time of the 1971 Population Census, has been followed to the end of 1983. All identified transmission installations within pre-defined areas were included in the study with the result that the greater part of the study group were believed to be resident near relatively low voltage sub-stations. Overall mortality was lower than expected and no evidence of major health hazards emerged. The only statistically significant excess mortality was for lung cancer (in women overall, and in persons living closest to the installations); this result is difficult to interpret in the absence of smoking data, and is not supported by other evidence but does not appear to be due to the social class distribution of the study group. The study did not support previously reported associations of exposure to electro-magnetic fields with acute myeloid leukaemia, other lymphatic cancers and suicide.