The multiple endocrine neoplasia (MEN) syndromes consist of three distinct disease entities. They have in common adenomatous, carcinomatous or hyperplastic involvement of a variety of endocrine glands, and an autosomal dominant inheritance. MEN I includes hyperparathyroidism, islet cell and pituitary tumors. The components of MEN IIa are hyperparathyroidism, medullary thyroid carcinoma and pheochromocytoma. MEN IIb includes multiple neuromas, medullary thyroid carcinoma and pheochromocytoma. Effective tests are available for the early detection of components of the syndromes in potentially affected patients. Screening can lead to therapeutic intervention before clinical sequelae ensue.