Background: This study assessed the long-term effects of cancer therapies on reproductive status in adult male childhood cancer survivors, evaluated the treatment-related risk factors for hypergonadotropic hypogonadism and assessed the association between the FSH levels and the later need for assisted reproductive techniques (ART). Methods: The study cohort included adult male 5-year survivors of childhood cancer who were treated in our institution between 1966 and 2003. Data concerning patient and treatment characteristics, FSH, LH and testosterone levels and pregnancy outcome were collected. Multivariate regression analyses were performed to evaluate the treatment-related risk factors for disturbances in reproductive endocrine status. The diagnostic and predictive values of FSH and later need for ART were evaluated. Results: Data on reproductive endocrine status were available for 488 survivors (86.4) of the 565 male survivors who visited the outpatient clinic in adulthood. The median follow-up time from initiation of treatment to first visit to the outpatient clinic in adulthood was 15 years. The prevalence rates of elevated FSH levels and decreased testosterone levels were 33 and 12, respectively. The use of procarbazine, cyclophosphamide, vinca-alkaloids, other alkylating agents, pelvic/abdominal irradiation, total body irradiation and testicular surgery were identified as treatment-related risk factors for elevated FSH levels. During the follow-up period, 73 men reported 120 conceptions, which resulted in 103 live births. Of these men, 56 (77) were able to achieve conception naturally. All men whose partners conceived by assisted reproductive techniques (n 13) had elevated FSH levels at their first visit after their 18th birthday (sensitivity: 100; 95 CI: 71100) and all male survivors with a normal FSH level did not need assisted reproductive techniques (negative predictive value: 100; 95 CI: 89100). Conclusions: One-third of young adult male survivors of childhood cancer has elevated FSH levels. FSH appears to be a very sensitive marker for the need of assisted reproductive techniques in male childhood cancer survivors.