Knowledge of the impact of different conditioning regimens used in bone marrow transplantation on spermatogenesis is important in pre-BMT counselling for three reasons: (1) Most young patients who have not had children are concerned with their subsequent fertility; (2) For a number of diseases there are competing therapeutic options that may affect spermatogenesis more or less seriously; (3) Since spontaneous recovery of spermatogenesis is rare, it would be necessary to offer cryopreservation as soon as possible after diagnosis and prior to any treatment. This retrospective study evaluates 99 semen samples obtained in 64 patients who underwent BMT between 1982 and 1996. Recovery of spermatogenesis was observed in 90% of patients conditioned with cyclophosphamide (CY), in 50% of patients with CY plus busulphan (BU) or thiotepa and in 17% of patients with CY plus total body irradiation (TBI) or thoracoabdominal irradiation (TAI). Sperm quality following CY was within the normal range (WHO) in the majority of patients, whereas it was consistently severely impaired in patients who received irradiation or two alkylating agents. Following CY, spermatogenesis recovery was observed in 60% of patients tested 1 year post transplant and it was accomplished within the third year in 80% of cases. Following CY + TBI/TAI recovery of spermatogenesis never occurred before the 4th year post transplant and was demonstrated as late as 9 years in one patient who was azoospermic 1 year earlier. No statistical correlation between age and recovery of spermatogenesis could be demonstrated. The overall high incidence of azoospermia (70.3%) supports the indication for semen cryopreservation in young patients undergoing BMT. These results have implications for semen sample timing before and after BMT and underline a need to collect further data through prospective multi-center studies.