To assess the gonadal effects of chlorambucil given to prepubertal and pubertal boys, testicular function of 21 adolescents or young men, treated with chlorambucil for nephrotic syndrome before or during puberty, have been evaluated. Evaluations have been based on penis and testicular size, serum FSH, LH and testosterone values, and sperm count. Nine patients have testicular hypotrophy, 13 have significantly elevated FSH, 17 have azoospermia, two have severe oligospermia, and repetitive sperm counts show no improvement. A quiescent state of the epithelium of the seminiferous tubules does not protect against chlorambucil toxicity, which seems to be related to the cumulative dose. Chlorambucil should be used with caution, even in children.