Since 1974, a total of 11 families with the multiple endocrine neoplasia syndrome type I (MEN-I), including 52 patients, were identified. Fifteen of these 52 patients died of MEN-I-related complications (mean age, 44 years), most of them in the period before screening was started. In 11 of the 15 patients, death was caused by complicated peptic ulcer disease, in 2 by metastasis of an endocrine pancreatic tumor, and in 2 by renal failure due to hyperparathyroidism. Family screening led to the diagnosis of 43 new endocrine lesions: 21 cases of hyperparathyroidism, 16 endocrine pancreatic tumors, and 6 pituitary tumors. Hyperparathyroidism either was the first manifestation of MEN-I or was diagnosed simultaneously with the other components of the syndrome in 44 (94%) of the 47 patients who underwent full evaluation.