In a follow-up study of 50,897 pregnancies, poliomyelitis and influenza immunizations, and viral infections were evaluated as possible risk factors for the development of malignancies in the offspring born between 1959 and 1966. Ascertainment of malignancies was based on clinical follow-up during the first year of life and on mortality experience covering the first four years of life. In 18, 342 children whose mothers were vaccinated during pregnancy with killed polio vaccine there were 14 malignancies (7.6 per 10,000) and in 32, 555 non-exposed children there were 10 (3.1 per 10,000). In the vaccinated group, nine malignancies occurred in children whose mothers were immunized during the first four lunar months of pregnancy (13.2 per 10,000). Time clustering of administration of the vaccine was evident in mothers whose children developed malignancies. There were seven tumours derived from neural tissue in the exposed children (3.8 per 10,000) and one in the non-exposed children (0.3 per 10,000). Elimination of three microscopic tumours reduced the overall rates in the exposed and non-exposed groups to 6.5 and 2.8 per 10,000, respectively. There was no evidence of an excess of malignancies in children exposed in utero to attenuated live polio vaccine, to influenza vaccine, or to spontaneous viral infections.