Functional inactivation of the retinoblastoma (RB) tumor suppressor gene is important in the pathogenesis of many human tumors. Recently, the frequent occurrence of pituitary tumors was reported in mice genetically engineered to have one defective RB allele, a genetic background analogous to that of patients with familial retinoblastoma. The molecular pathogenesis of human pituitary tumors is largely unknown, and the potential role of RB gene inactivation in these neoplasms has not been examined. Consequently, we studied 20 human pituitary tumors (12 clinically nonfunctioning tumors, 4 somatotroph adenomas, 2 prolactinomas, and 2 corticotroph adenomas) for tumor-specific allelic loss of the RB gene using a highly informative polymorphic locus within the gene. Control leukocyte DNA samples from 18 of these 20 patients were heterozygous at this locus, permitting genetic evaluation of their paired tumor specimens. In contrast to the pituitary tumors in the mouse model, none of these 18 human tumors exhibited RB allelic loss. These findings indicate that RB gene inactivation probably does not play an important role in the pathogenesis of common types of human pituitary tumors.