Delayed childbearing is a common phenomenon in industrialized countries. This review focuses on age-associated alterations of male fertility and genetic risks. Semen volume, sperm motility and sperm morphology decrease with age, whereas the data concerning sperm concentration are conflicting. The age-related changes of semen parameters reflect the histological modifications which are found to varying degrees in individual testes. Men aged >40 years contribute to reduced fertility and fecundity of a couple, especially when the female partner is also of advanced age. Because relatively few children are born to older fathers and genetic diseases are rare, there is little statistical power supporting an association of genetic diseases in the offspring with advancing paternal age. Nevertheless, autosomal dominant diseases and some diseases of complex aetiology, such as schizophrenia, are associated with advancing paternal age. The single point mutations in sperm which are responsible for achondroplasia and Apert's syndrome, two autosomal dominant diseases, increase with the man's age. In case of Apert's syndrome this increase is believed to be due to a pre-meiotic selection of mutant spermatogonia. Although structural chromosome anomalies and disomies of certain chromosomes increase in sperm with the man's age, paternal age is, with the exception of trisomy 21, not associated with numerical or de novo structural chromosomal aberrations in newborns. However, even if the genetic risk for progeny from older fathers is slightly increased, the risk to the individual is low.