Of a group of 79 patients (45 males, 34 females) with myelomeningocele (MMC), 52 had associated hydrocephalus. Three of the hydrocephalic patients (two arrested and one shunted) were found to have precocious sexual development. Endocrine investigations confirmed true isosexual precocity. Hydrocephalus is known to be associated with precocious puberty, but the occurrence of sexual precocity in patients with hydrocephalus in conjunction with MMC has not been described to date. As the clinical diagnosis of hydrocephalus in a young child is often unreliable, routine computerized tomographic scans of all MMC patients is advised, and even patients with arrested hydrocephalus should be followed carefully for signs of precocious puberty. In addition, a high incidence (15%) of cryptorchidism was found in the group of MMC patients reviewed.