An increasing proportion of boys and young men with cancer will survive their disease and desire fertility. Unfortunately, the cancer treatment, and in some cases the malignant disease itself, may have a negative and permanent impact on the individual's fertility potential. This effect is highly dependent on the type and dose of therapy as well as the age at which it has been given. Basic knowledge in this field is necessary to enable oncologists and fertility specialists to counsel these patients about their fertility prospects and, if appropriate, advise them to take precautions (e.g. the cryopreservation of semen) to safeguard their fertility. Another aspect of the relationship between cancer and infertility is the possibility that men with testicular dysfunction may have an increased risk of testicular cancer. Screening for early testicular malignancy may therefore be advisable in some groups of men with poor semen quality.