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Objective: The purpose of this study was to examine the risk of infertility that is associated with paternal age, because this factor rarely has been investigated, whereas maternal age of >or=35 years is a well-known risk factor., Study Design: This large, retrospective, population-based sample included 6188 European women (from Denmark, Italy, Spain, Germany) aged 25 to 44 years who were selected randomly from census registers in 1991 through 1993., Results: Among couples composed of a woman aged 35 to 39 years, risks were significantly higher when paternal age was >or=40 years than when paternal age was <40 years, with an adjusted odds ratio of 2.21 (95% CI, 1.13, 4.33) for delay in pregnancy onset (failure to conceive within 12 months) and of 3.02 (95% CI, 1.56, 5.85) for difficulties in having a baby (failure to conceive within 12 months or pregnancy not resulting in a live birth)., Conclusion: Like maternal age of >or=35 years, paternal age of >or=40 years should be considered to be a key risk factor for infertility.