Background: The objective of this study is to investigate the influence of male age on human fertility, defined as the birth rate for a given population. Methods: Data from the Spanish National Statistics Institute (INE) for the year 2004 from a total of 454,753 newborn infants and sorted by male and female age groups were evaluated. In order to correct the influence of female age-related fertility, a different analysis was performed considering only women under 30 years of age. Results: From a demographic point of view, male fertility started to decline at 35–39 years of age. This decline is constant and follows an exponential pattern (slope –0.11 to –0.12). The trend persisted when the data were adjusted for every 1,000 men in the age group, as well as when only women under the age of 30 were considered. Male fertility showed a 21–23% annual decrease starting at the age of 39. Conclusion: An exponential decrease in human fertility which is independent of the woman’s age was observed with male aging. This decay is probably due to a downfall in male fecundity, closely related to a decline in sperm quality. However, social or behavioral causes for this trend cannot be excluded.