In a prospective, randomized, controlled trial, we evaluated the efficacy of intrauterine insemination (IUI) in 54 couples with cervical factor (CF) infertility and in 19 couples with male factor (MF) infertility. The diagnoses were confirmed by strict criteria and all other etiologies of infertility were ruled out prior to entry. Patients served as their own controls with alternating cycles of washed sperm IUI and whole-ejaculate intracervical insemination (ICI). In 113 paired cycles (mean 2.1 pairs/patient; range 1-7 paired cycles), 13% (n = 7) of CF patients became pregnant by IUI, 7% (n = 4) by ICI, and 6% (n = 3) by intercourse after missing inseminations. Seventy-one percent of IUI pregnancies occurred in the first 2 months. In 45 paired cycles for MF patients (mean 2.4 pairs/patient; range 1-6 paired cycles), two patients conceived, both in the first IUI cycle. These data suggest that pregnancies resulting from IUI occur during early treatment cycles.