A study of reproductive function was carried out in 31 patients following long-term treatment of acute lymphoblastic leukemia (mean: 4 1/12 years); 17 prepubertal; nine pubertal, and five adults. Spontaneous clinical pubertal development was observed in 4 of 17 prepubertal patients. Serum testosterone response to hCG was normal or increased in the nine prepubertal patients studied. Intratesticular testosterone concentration was normal in 11 out of 12 patients. Only two of 14 prepubertal patients had a reduced number of germ cells. Normal progress of spermatogenesis was seen in six out of seven pubertal patients and only moderate hypospermatogenesis with focal hyalinosis was found in the five adults. Two of them had normal sperm count and one fathered two children. In six out of 14 prepubertal patients unexpected early signs of pubertal stimulation were found. Serum gonadotrophins response to LH-RH was normal in most patients. Testicular infiltration of blastic cells was found in four of 26 biopsies, but only one of 23 patients was without clinical signs of testicular relapse. Computerized axial tomography was normal in 10 out of 13 patients. Bone age was significantly below chronological age in four out of 15 patients. This study suggests that the gonad is moderately injured by long-term antileukemic therapy. In addition, microscopic testicular infiltration is not frequent in subjects without testicular enlargement.