Chemotherapy has considerably improved the prognosis of solid tumours in children, but may have very adverse effects, particularly on fertility. A study was conducted at the Gustave Roussy Institute to identify the toxic effect of chemotherapy on male fertility. At present, 205 patients, treated during childhood have entered the study. Basal FSH-LH have been assayed to assess possible germ cell damage although azoosperia can not be eliminated. Results were normal in 127 patients (62%) and increased basal FSH levels were found in 78 (38%). Endocrine function was not altered: all patients were either impubertal or intrapubertal at diagnosis and subsequently achieved normal puberty. Multivariate analysis revealed an obvious toxic effect of 2 alkylating drugs: cyclophosphamide and procarbazine. No toxic effect was observed for vincristine, dohorubicin or actinomycin D. Age and pubertal status at diagnosis were not correlated with toxic effects. At present, no conclusion for other drugs may be made but results high dose metotrexate are promising. For lomustine and cisplatin, less favourable, though nonsignificant, results have been obtained. Complete recovery is possible several years later.