From the Ontario Perinatal Mortality Study and the British Perinatal Mortality Survey, 89,302 babies surviving at least 7 days served as a base population (or a prospective study of the relationship between smoking in pregnancy and cancer in the offspring. The smoking habits of the mother during pregnancy were recorded before or just after the birth of the child. In the two base populations combined, there were 65 cancer deaths and 32 cancer survivors in the period from birth to a minimum of 7 and a maximum of 10 years of age. For cancer of all sites, the children of smokers had a relative risk of 1.3 with 95% confidence limits of 0.8–2.2. Evidence of a dose-response relationship was lacking. Although these results make it most unlikely that in utero exposure to tobacco smoke has a broadly carcinogenic effect on the fetus, a response confined to one tissue or expressed over a narrow age range cannot be ruled out.