Our objective was to investigate if there was (i) an excess risk of leukaemia/non-Hodgkin's lymphoma among children of male radiation workers at the Sellafield nuclear installation in Cumbria, northwest England; (ii) a dose-response relationship between fathers' preconceptional irradiation and their children's risk of leukaemia/non-Hodgkin's lymphoma; and (iii) whether any observed association could be explained by demographic factors. We performed a cohort study of live births, 1950-1991 in Cumbria, followed up to age 25 years or the end of 1991, comparing the risk of leukaemia/non-Hodgkin's lymphoma among all 9,859 children of male radiation workers to that among all 256,851 children of non-Sellafield fathers. Children of radiation workers had a higher risk of leukaemia/non-Hodgkin's lymphoma than other children [rate ratio (RR) = 1.9, 95% confidence interval (CI) 1.0-3.1, p = 0.05]. Adjustment for population mixing greatly reduced the excess risk in the village of Seascale, adjacent to Sellafield, but had little effect elsewhere. The risk increased significantly with father's total preconceptional external radiation dose (RR(100mSv) = 1.6, 95% CI 1.0-2.2, p = 0.05). This dose-response was not reduced by adjustment for population mixing. Although our 13 exposed cases included 10 considered previously (Gardner et al., BMJ 1990;300:423-34), we used a cohort rather than a case-control design, with wider temporal and geographic boundaries, and confirmed the statistical association between father's preconceptional irradiation and child's risk of leukaemia/non-Hodgkin's lymphoma that they reported. The possibility remains that paternal preconceptional irradiation may be a risk factor for leukaemia/non-Hodgkin's lymphoma, and this effect may not be confined to Seascale.