Acoustic neurilemmomas are more frequent, larger, and more vascular in women, and such lesions have a rapidly progressive clinical course in pregnant women; these findings suggest that the growth of these neoplasms may bear a relationship to certain hormone levels. Tissues from 8 patients with acoustic neurilemmoma (3 men and 5 women) were studied by a new fluorescent steroid histochemical technique to detect the presence of estrogen receptors in or on neoplastic cells. Estrogen receptor protein has also been found in meningioma cells in women. Neurilemmoma of the acoustic nerve is the second neoplasm of the central nervous system in which such receptors have been demonstrated.