Prospective randomization of 60 couples with unexplained infertility was performed for treatment either with intrauterine insemination (IUI), using a volume of 0.5 ml of the inseminate, or Fallopian tube sperm perfusion (FSP), using a volume of 4 ml of inseminate. The protocols for ovarian stimulation and induction of ovulation were the same in the two groups. The two groups were similar concerning age of the female at the start of treatment and the number of follicles > 15 mm diameter, the serum oestradiol concentrations and the endometrial thickness on the day of human chorionic gonadotrophin (HCG) administration. The mean (+/- SD) number of motile spermatozoa inseminated was significantly higher in the FSP group than in the IUI group (52 +/- 5 x 10(6) and 28 +/- 3 x 10(6) respectively). In the FSP group, 30 women were given a total of 52 treatment cycles; 14 clinical pregnancies occurred in this group, giving a pregnancy rate of 26.9% per cycle and 46.7% per woman. In the IUI group, 28 women were given a total of 51 treatment cycles; five clinical pregnancies occurred, giving a pregnancy rate of 9.8% per cycle and 17.9% per woman. The pregnancy rates per cycle and per woman in the FSP group were significantly higher than in the IUI group (P < 0.05, chi-square test). This study indicates that in the treatment of couples with unexplained infertility, Fallopian tube sperm perfusion (FSP) is more successful than intra-uterine insemination (IUI).