The twinning incidence from 1925 onwards was investigated in Campinas, State of Sao Paulo, Brazil. This study has shown that the decline of the twinning rate is a phenomenon that started in this city much before the introduction of oral anovulatories in the sixties. The decline in twinning from 1925 to 1965 (42%) seems to be due mostly to a striking decrease (60%) in the proportion of Negroid parturients, together with the decline of parity (9%) of Caucasoid women. Maternal age did not influence twinning incidence from 1925 to 1965. The decline of the incidence of twin births after 1965 in Campinas was influenced by the decrease of parity, of maternal age and of the prevalence of Negroid women in the population.163793801