A review of 106 patients with multiple endocrine neoplasia (MEN) type 1 reported between 1966-1989 in Japan was conducted in order to clarify the natural history of this disease. Sporadic MEN 1 was found in 61 patients, and familial MEN 1 was found in 45 patients from 15 families. The mean ages at diagnosis of the two groups were 46.2 and 41.3 years, respectively, and the male to female ratio was 3:4. With regards to the involvement of the pituitary, parathyroid and endocrine pancreas, the combination of three endocrine glands was 31%, and that of two was 48%, in which the pituitary, parathyroid and endocrine pancreas had tumorous lesions in 60%, 88% and 63%, respectively. The first clinical manifestations of MEN 1 were the symptoms of hyperparathyroidism (32%), pituitary tumors (26%), peptic ulcer (28%) and hypoglycemia (13%). These symptoms appeared between 6 and 57 years of age (mean 34 years). The main clinical symptoms of the pituitary tumors were acromegaly and gigantism (37%), galactorrhea-amenorrhea syndrome (20%), Cushing's disease (10%) and visual disturbance due to compression of the tumor (20%). The clinical manifestations of hyperparathyroidism were mainly asymptomatic hypercalcemia (41%), nephrolithiasis (42%) and osteitis fibrosa (5%). In the patients with pancreatic tumor, Zollinger-Ellison syndrome (52%) and hypoglycemic symptoms (42%) were found. Tumors in the adrenal cortex, thyroid, carcinoid and lipoma were detected in association with MEN 1 in the frequencies of 28%, 21%, 9% and 5%, respectively. Death was reported in 37 patients between 9 and 86 years of age (mean 50.0 years). The cause of death was gastrointestinal bleeding and perforation (45%), surgery (24%) and disseminated carcinomatosis (18%). Recently, the gene predisposing to this syndrome has been assigned to chromosome 11 (11q13) in non-Japanese cases but not yet in Japanese MEN 1 patients.