Germline mutations in the RET proto-oncogene are seen in the majority of patients with the dominantly inherited cancer syndromes multiple endocrine neoplasia type 2 (MEN 2). The clinical subtypes of MEN 2 (MEN 2A, MEN 2B and familial MTC) all have medullary thyroid carcinoma, but vary in the involvement of pheochromocytoma, parathyroid adenoma/hyperplasia and developmental abnormalities. A single RET mutation, resulting in the substitution M918T, has been identi®ed in 94% of cases of MEN 2B (which consists of MTC, pheochromocytoma and developmental abnormalities). Here we report the identi®cation of a new germline RET mutation (A883F) in two de novo cases of MEN 2B. Identi®cation of this new mutation will contribute to understanding the molecular basis of MEN 2B, and will assist in the clinical management of families harbouring this mutation.