The objectives of this study were to examine the predictive value of the PCT in relation to treatment-independent conception rates, to determine which cutoff point best predicted fertility outcome, and which form of spermatozoal motility on PCT is most significantly related to fertility outcome. A prospective study was undertaken of couples referred for the investigation of "unexplained" infertility. One hundred and twenty six couples were recruited between July 1984 and April 1986. On further investigation female infertility factors were identified in 32 (25%) of couples, who were therefore excluded from the study. All conceptions were treatment independent. The relationship between the result of the postcoital test and the chance of conception was studied using a modification of Cox's regression model of life table analysis. The mean length of involuntary infertility prior to referral was 75 months (range = 18-192, SD = 39.2). The mean length of follow-up was 18 months (range = 1-34, SD = 8.4). A 98% follow-up rate was achieved. Eighteen women conceived, giving a 32-month treatment-independent conception rate of 22%. A cutoff point of one spermatozoon exhibiting sluggish motility per HPF was the most effective method of classifying the results of the postcoital test (X2(1) = 4.28, P = 0.037, RR = 4.7. This would suggest that the most efficient form of spermatozoal motion in cervical mucus is slow or sluggish motility.