This retrospective case-control study of 56 cases and 56 controls measured extra low frequency (ELF) electric and magnetic fields between 2000 h and 0800 h in the bedplaces of children with leukaemia. Mean ELF electric field (E-field) levels found in case homes of 13.9 Vm-1 (SD: 13.6) were significantly higher (P < 0.01) compared with only 7.3 Vm-1 (SD: 12.9) in controls matched for age and sex. Moreover, applying conditional logistic regression, a dose-response relationship emerged between E-field exposure and incidence: above 20 Vm-1 the relative risk was 4.69 (95% CI: 1.17-27.78; P = 0.025), whereas at levels of 10-19 Vm-1 it was 2.40 (95% CI: 0.79-8.09) and at levels of 5-9 Vm-1 it was only 1.46 (95% CI: 0.47-5.10). By contrast, similar readings of the rms ELF magnetic field found no significant case-control differences: mean levels in cases' homes of 0.070 microT (SD: 0.070) compared with 0.057 microT (SD: 0.038) in controls. Although there were imperfections in the study design, it is concluded that the importance of the E-field may have been overlooked in epidemiological studies to date.