Macroorchidism is a rare condition in children and is usually associated with fragile X syndrome. Other possible etiologies of macroorchidism are long-standing primary hypothyroidism, adrenal remnants in congenital adrenal hyperplasia, follicle stimulating hormone (FSH)-secreting pituitary macroadenomas, local tumors, lymphomas, and aromatase deficiency. Bilateral macroorchidism can be a normal variant in adult men. We report the case of an 11.5-year-old boy who was referred to our hospital for evaluation of marked bilateral testicular enlargement in the previous few months. Physical examination revealed a testicular volume larger than 30 ml. Complementary investigations allowed us to rule out all hitherto reported etiologies of bilateral macroorchidism and therefore a diagnosis of idiopathic macroorchidism was established. The increased number of Sertoli cells found on biopsy, without elevated plasma FSH levels, prompted us to speculate about a hypothetical FSH receptor hyperfunction as a possible cause of the exaggerated testicular enlargement in this patient.