Objective: To investigate the risk of cancer in children living close to overhead power lines with magnetic fields of > or = 0.01 microteslas (microT).
Design: Cohort study.
Setting: The whole of Finland.
Subjects: 68,300 boys and 66,500 girls aged 0-19 years living during 1970-89 within 500 m of overhead power lines of 110-400 kV in magnetic fields calculated to be > or = 0.01 microT. Subjects were identified by record linkages of nationwide registers.
Main outcome measures: Numbers of observed cases in follow up for cancer and standardised incidence ratios for all cancers and particularly for nervous system tumours, leukaemia, and lymphoma.
Results: In the whole cohort 140 cases of cancer were observed (145 expected; standardised incidence ratio 0.97, 95% confidence interval 0.81 to 1.1). No statistically significant increases in all cancers and in leukaemia and lymphoma were found in children at any exposure level. A statistically significant excess of nervous system tumours was found in boys (but not in girls) who were exposed to magnetic fields of > or = 0.20 microT or cumulative exposure of > or = 0.40 microT years.
Conclusions: Residentia magnetic fields of transmission power lines do not constitute a major public health problem regarding childhood cancer. The small numbers do not allow further conclusions about the risk of cancer in stronger magnetic fields.