In a study of 16 different families, 55 persons were considered to be affected by the syndrome of multiple endocrine neoplasia type 1 (MEN-1). Serum calcium measurements were available for all but six patients, long deceased, and hypercalcemia was invariably present. All but four of these patients have been subjected to surgery, and hyperparathyroidism was verified in each case. In all the patients studied, hyperparathyroidism was apparently the first manifestation of the MEN-1 syndrome. Pituitary tumors were detected in 23 patients, and there was evidence of pancreatic lesions in 32. Although either the pituitary or pancreatic manifestations dominated clinically in some patients, hyperparathyroidism was invariably present when sought. Measurement of serum calcium in asymptomatic relatives at risk for MEN-1 disclosed no case of hyperparathyroidism in children below the age of 18, whereas half of all screened persons between the ages of 18 and 30 were found to have hyperparathyroidism. In some, the serum calcium values were only marginally or even intermittently elevated. It is concluded that since hyperparathyroidism is apparently the first lesion in MEN-1, screening for this syndrome should primarily be directed toward the diagnosis of hyperparathyroidism. The temporal relationships suggest that hyperparathyroidism could be a prerequisite for the other types of endocrine neoplasms in the MEN-1 syndrome.