Sex differences in language performance have long been noted, with females more verbal and males superior in visual-spatial tasks. Two theories seek to explain the differences in language function. Waber (1976, Science, 193, 572-574) suggests that these sex differences are secondary to differences in bilateral language function related to the faster maturation rate in girls. Geschwind and Galaburda (1985, Archives of Neurology, 42,(I), 428-459; (II), 521-552; (III), 634-654) on the other hand posit an intimate interrelationship of sex hormones, the immune system, and laterality as influencing the ultimate asymmetry of the nervous system, which in turn could account for such differences. In the present study, language function was examined in patients with accelerated maturation caused by conditions with sex hormone elevation (idiopathic precocious puberty and congenital adrenal hyperplasia). The degree of maturational advancement was similar between the two groups. However, significant language performance differences were noted between androgen- vs. estrogen-exposed patients, regardless of genetic sex or diagnosis of the patient, indicating a hormonal effect on language development over time. These data support Geschwind and Galaburda's multifactorial theory for the origin of sex differences in language performance, and argue against Waber's maturational hypothesis.