Twenty-three prepubertal subjects treated for Wilms' tumor (10 males and 13 females) were endocrinologically evaluated off therapy from 0.5 to 4.08 years. They were divided into two groups: 11 subjects (6M, 5F) who had received chemotherapy only (group 1) and 12 (4M, 8F) who had in addition received abdominal radiation (1,500-3,000 rads) (group 2). Follicle-stimulating hormone (FSH), luteinizing hormone (LH), prolactin (PRL), thyroid-stimulating hormone (TSH), free thyroxine (FT4), free tri-iodo thyronine (FT3), testosterone (T), estradiol-17 beta (E2), and cortisol (F) were measured by radioimmunoassay (RIA). Plasma levels of TSH, PRL, FT4, FT3, and F were normal in both groups, as were FSH, LH, T, and E2 in group 1. In group 2, female subjects showed FSH levels significantly higher than controls, while LH and E2 were normal; male subjects showed significantly higher LH levels, while FSH and T levels were normal. These results indicate that in the treatment protocol used by us for Wilms' tumor (WT), chemotherapy does not affect endocrine function, whereas abdominal radiation seems to damage gonadal function directly. The present findings indicate that gonadal damage may be revealed in WT before puberty not only in females, as has been previously reported, but also in male subjects.