Many different aetiologies for childhood cancer have been suggested, but few are well established. One is that parental autoimmune disease is linked with susceptibility for haematopoietic malignancies in their offspring during childhood. The present study is the first to investigate this hypothesis using a follow-up design. A cohort of 53,811 children of more than 36,000 patients diagnosed with a systemic, organ-specific or suspected autoimmune disease were followed up for cancer incidence in the Danish Cancer Registry during 1968-1993. The parents were identified through the National Registry of Patients, while their children were traced in the Central Population Register. Cancer incidence among the offspring was compared with that in the corresponding childhood population of Denmark. In total, 115 cancers were observed among children aged 0-19 years, yielding a non-significant standardized incidence ratio of 1.07. Lymphomas contributed 21 cases to the overall number of tumours, 60% more than expected (95% confidence interval (CI) 1.0-2.4); leukaemia contributed 37 cases representing an excess of 30% (95% CI 0.9-1.8). Our results give some support to the hypothesis that parental autoimmune disease is associated with childhood lymphoma and leukaemia.