Sixteen patients affected with multiple endocrine neoplasia, type 2b (MEN 2b), were evaluated by clinical, neurological, nerve conduction and electromyographic, and postmortem examinations. Eight of the 11 patients examined clinically had symptoms: 5, neurogenic constipation; 1, failing vision due to hypertrophied corneal nerves; 1, neuromuscular symptoms and pes cavus; and 1, facial disfigurement. Expression of the dominantly inherited MEN 2b gene is more variable than previously known. When neuromuscular findings are present alone, the features may be those of peroneal muscular atrophy. Because 10 of the 11 patients had sufficiently full expression of the dominantly inherited gene--"Marfanlike" body build, full and fleshy lips, whitish yellow nodules (neuromas) on the tip and edges of the tongue, pes cavus, or peroneal muscular atrophy--the presence of MEN 2b was recognized and a search for the usually associated medullary thyroid carcinoma was instigated. In addition to the recognized involvement of autonomic nerves, we have confirmed that somatic motor and senory neurons may be involved. Findings at postmortem evaluation indicate that symptoms can be attributed to neuroma formation: a characteristic adventitious plaque of tissue composed of hyperplastic, interlacing bands of Schwann cells and myelinated fibers overlay the posterior columns of the spinal cord.