Neoplasms of the hypothalamus represent a group of tumors with very interesting and variable clinical symptomatology. Although the available literature concerning tumors and malformations affecting the function of the hypothalamohypophyseal system is extensive, our case represents interesting clinical aspects and unique histological characteristics. Report of Case Clinical History. —The patient, a white woman, was born in 1925 of normal parentage and had the usual childhood diseases. Menarche was at the age of 14, with regular 30-day cycles. At the age of 19 she abruptly became amenorrheic and lost 15 lb. (from 124 to 109 lb.) Thereafter she began gaining weight; one year later her weight was 160 lb., and within a few years it had increased to 260 lb. Thirst was increased; she drank two glasses of water every 45 minutes and developed polyuria and nocturia (micturition four to five times every night); this was accompanied by polyphagia and headaches.