Epidemiologic studies were made of familial medullary thyroid carcinoma-pheochromocytoma syndrome occurring in an autosomal dominant pattern in 20 members of a kindred, including 13 with occult tumors diagnosed by laboratory surveillance. Parathyroid hyperfunction, neurocutaneous lesions, dermatoglyphic aberrations, serologic hyperreactivity to Epstein-Barr virus, and increased in vitro viral transformation of skin fibroblasts were detected in some family members; the significance of these findings may be clarified by careful evaluation of these indIviduals for new tumors.