OBJECTIVE: Cervical mucus is vital in the regulation of sperm survival and transport through the reproductive tract. The goal of this study is to assess whether the lowered fertility for men in their late 30s and early 40s is related to the nature of cervical mucus on the day of intercourse. METHODS: In a prospective study of 7 European family planning centers, 782 couples not using birth control recorded daily observations of intercourse and the nature of cervical mucus. Using data from 1,459 menstrual cycles, 342 ending in pregnancy, we estimate day-specific conception probabilities in relation to mucus and male and female age. RESULTS: On days where cervical mucus was not evident, intercourse for men in their late 30s and early 40s was 50% less likely to result in a clinical pregnancy, adjusting for intercourse timing and female age. As secretions become more conducive to sperm transport, the effect of male age diminishes steadily from 21% on days with damp secretions, to 11% on days with thick mucus, to only 4% on days with most fertile-type mucus. CONCLUSION: The effect of male age on fecundability can be minimized by timing intercourse on days with optimal secretions. LEVEL OF EVIDENCE: II-2.