Prior experience in kindreds with a codon 891 RET protooncogene mutation indicates that carriers of this mutation develop only hereditary medullary thyroid carcinoma without evidence of other manifestations of multiple endocrine neoplasia type 2. In this paper, we report the first documented case in which medullary thyroid carcinoma and pheochromocytoma were clinically expressed in members of a family affected by the codon 891 RET mutation. Genetic analysis of the RET protooncogene in this family revealed an exon 15 missense mutation at codon 891 that resulted in a serine to alanine amino acid substitution. These findings indicate that patients with this mutation should be screened for pheochromocytoma.