The prognostic power of the post-coital test for natural conception depends on duration of infertility.

Controversy about the value of the post-coital test (PCT) has prompted us to re-analyse data from 207 couples, originally studied between 1982 and 1983, with at least 12 months' infertility at presentation, complete diagnostic information and exclusion of female factors, to clarify the effect of duration of infertility on the prediction of conception. In couples with less than 3 years infertility and a positive PCT, 68% conceived within 2 years compared with 17% of those with a negative result. After 3 years, corresponding rates were 14% and 11%. The relative risks of conception [95% confidence interval (CI)] calculated using the Cox's proportional hazards model were 0.23 (0.12-0.43) for a negative PCT (reference positive PCT) and 0.25 (0.13-0.51) for more than 36 months infertility (reference 12-23 months). Semen analysis had no extra predictive power given the duration of infertility and the PCT. The PCT is an effective predictor of conception where defined female causes of infertility are absent and duration of infertility is less than 3 years. Once infertility is prolonged (beyond 3 years) the conception rate is low even with a positive test because a large proportion of couples remaining childless so long have true unexplained infertility. Use of the PCT will enable clinicians to allocate scarce, expensive and invasive resources effectively.