Among 2,106 couples registered in 12 Canadian infertility clinics, 470 (22.3%) were classed as unexplained infertility after a uniform evaluation of the male ejaculate, ovulation, and tubal patency. The unexplained group included more older female partners; 44% were over 30 years of age at registration in the participating clinics, compared with 36% in other infertility diagnostic groups. The mean duration of infertility was 40.1 months, and the cumulative pregnancy rate was 36.6 +/- 2.9% at 2 years after registration. When the variables were examined with the use of proportional hazards analysis, each additional month of duration of infertility reduced the expected prognosis by 2%, and a history of pregnancy in the partnership improved the prognosis by 80%. Among couples with 3 years or more duration of infertility (cumulative pregnancy rate, 27.5 +/- 3.9%), an additional year in the age of the female partner when conception was first attempted (mean, 26.8 years) reduces the prognosis by 9%.