To evaluate the clinical significance of in vivo and in vitro testing of sperm ability to penetrate cervical mucus (CM), postcoital testing (PCT) and in vitro sperm-cervical mucus penetration testing were compared in a prospective study. Both in vivo and in vitro tests were standardized and performed after an oral course of estrogen therapy. Crossed in vitro sperm-cervical mucus penetration test, evaluated in 277 couples with CM of patients' wives and additionally with CM and semen of fertile donors, revealed that the male factor contributed to a significantly higher extent to deficient sperm-mucus interaction than the cervical factor. The overall pregnancy rate after 6 months was 23% (64/277). Whereas the outcome of PCT did not significantly predict subsequent fertility (PCT good pregnancy rate 24%/PCT poor 20%), significant differences were found for the sperm-cervical mucus penetration test with CM of patients' wives (pregnancy rate, 30.5% versus 8.5%) and for in vitro testing with donors' CM, but not for the mucus penetration test with donors' spermatozoa. Routine sperm analysis did not prove to be of prognostic value for a subsequent pregnancy. The results suggest that the in vitro sperm-cervical mucus penetration test is a good parameter of sperm function and, in particular, when performed as a cross-matching penetrability test, a valuable adjunct to PCT with regard to fertility prognosis.