Biochemical intake markers show that the fetus and breastfeeding infant are exposed to compounds in tobacco smoke if the mother smokes or is exposed to environmental tobacco smoke (ETS). Experimental studies demonstrate that some compounds in tobacco smoke are transplacental carcinogens. The available epidemiological data provide no conclusive evidence of an effect of maternal smoking during pregnancy on the risk of cancer in children. Only a few studies have been performed on ETS and cancer risks in adults, except for lung cancer, and no firm conclusions can be based on the results. There is a need for further epidemiological studies on passive smoking and cancer, both in children and in adults.