Time-specific conception rates were studied prospectively after a carefully validated post-coital test (PCT) in a consecutive series of 80 ovulatory women without any pelvic or seminal cause for their previous infertility. The PCT was performed on endocervical mucus collected 6-18 h after intercourse. The definition of a negative result, indicated by the absence of forward-moving sperm, depended on the finding being repeated in a second cycle and on the presence each time of fully developed mucus, indicated simply by its abundance, ductility and clarity. The time to conception was inversely related to the number of motile sperm seen. Simpler analysis showed a fivefold greater chance of conception associated with a positive compared with a negative PCT; after 2 years the cumulative conception rates were 84 and 16% respectively.