The molecular pathogenesis of granulosa cell tumors of the ovary is not understood, although recent studies have shown that immunoreactive inhibin secretion by these tumors may be used as a tumor marker. Granulosa cell tumors exhibit many features of normal granulosa cells, including a response to FSH and inhibin secretion. FSH levels are suppressed in patients with inhibin-secreting granulosa cell tumors, suggesting FSH-independent growth of these tumors. Activating mutations of the FSH receptor might, therefore, be involved in tumorigenesis. We sought to identify mutations in the FSH receptor genes of these tumors using PCR to amplify the exon encoding the transmembrane and cytoplasmic domains from the tumor DNA. Analysis of the amplicons for single strand conformational polymorphisms and direct sequencing confirmed a previously reported polymorphism in the C-terminal region of the receptor, but did not identify tumor-specific missense mutations and/or polymorphisms. In addition, ribonucleic acid from 3 granulosa cell tumors was used to confirm expression of the FSH receptor; expression was unexpectedly also observed in several ovarian mucinous cystadenocarcinomas used as controls. In conclusion, our failure to identify activating mutations of the FSH receptor in 15 granulosa cell tumors argues against a role for the FSH receptor in tumorigenesis and suggests that some subsequent component of this signal transduction pathway may be activated.