In a case-control study of childhood cancer a dose-response relationship was found between the number of cigarettes smoked per day by the mother during pregnancy and cancer risk in the offspring. When all tumour sites were considered the cancer risk was 50% higher for the most exposed group than for the controls. The risk was doubled for non-Hodgkin lymphoma, acute lymphoblastic leukaemia, and Wilms' tumour. These findings provide further evidence for the harmful effects of cigarette smoke on the growing fetus.