We evaluated gonadal function in 18 female and eight male patients with galactosemia due to transferase deficiency; it was normal in the males, but 12 females had signs of hypergonadotropic hypogonadism. All female patients had a 46,XX karyotype, normal levels of thyroid hormone and prolactin, and no anti-ovarian antibodies. The biologic activity of urinary gonadotropins was normal. Ultrasonography of the pelvis revealed that ovarian tissue was diminished or absent. Total estrogens increased in one of two patients after administration of human menopausal gonadotropin. The frequency of hypergonadotropic hypogonadism was higher in females in whom dietary treatment for galactosemia was delayed. Clinical course and mean erythrocyte galactose-1-phosphate and urinary galactitol levels did not correlate with ovarian function. We conclude that female patients with galactosemia have a high incidence of ovarian failure due to acquired ovarian atrophy. Galactose or its metabolites may be toxic to the ovarian parenchyma, particularly during the immediate neonatal period.