A uniform increase has been observed in the incidence of extrauterine pregnancies in Denmark from 11.7 per 1,000 known conceptions in 1981 to 16.9 per 1,000 known conceptions in 1986. This increase is significant. The incidence of extrauterine pregnancies increases with increasing maternal age until the age of 40 years. The main increase occurred in the age group 20-39 years, where the increase is significant. The same age-related tendency has been demonstrated regionally and in Denmark as a whole. Where all Danish counties are concerned, an increasing frequency of extrauterine pregnancies was demonstrated from 1981 to 1986. The risks are greater in urban regions than in the rural regions. The etiology of the increase at a national level and the age-related and regional variations are unknown.