The purpose of the present study was to establish the occurrence of infertility in Finnish women aged 30, 35 and 40 years. The study is based on an interview by questionnaire carried out on women participating in screening for cancer by Pap smear. The participation rate is generally about 60%. A total of 4,879 women were interviewed between May 1981 and April 1982. 149 incomplete replies were discarded and an additional 528 women not at risk of conception were excluded. The final analysis included 4,202 women. Actual infertility, i.e., failure to establish pregnancy at the time of interview, occurred in 438 women (10.4%). The overall infertility rate was 15.4%. The primary infertility rate (5%) seems to be similar in all the age groups interviewed. Secondary infertility increased among women aged 30-40 years, from 4.1 to 9.9%. Actual infertility increased highly significantly between the age groups 30 and 40 years. A limited number of women are permanently infertile, but the percentage of those who at some point in their lives are involuntarily childless is higher.