Carcinoids occur in association with MEN types 1 and 2. To determine the relationship between carcinoids and MEN, we reviewed nine patients with carcinoids and other endocrine tumors. Analyzing these 9 patients and 56 other patients previously described in the literature, we found several clinically important relationships. In contrast to the usual midgut and hindgut origin, most carcinoids associated with MEN (69 percent) are of foregut origin (thymus 24 percent, bronchus 27 percent, stomach 3 percent, and duodenum 14 percent). Carcinoids are more commonly associated with MEN type 1 than MEN type 2 (59 patients and 6 patients, respectively). Thymic carcinoids associated with MEN are more common in men (15 versus 2), and most (82 percent) are malignant. Bronchial carcinoids associated with MEN are more common in women (15 versus 4), and most (74 percent) are benign. There is a strong association between thymic carcinoids and parathyroid tumors and between bronchial carcinoids and pituitary tumors. Most patients with carcinoids and hyperparathyroidism (82 percent) have had parathyroid hyperplasia or multiple parathyroid adenomas. Thus, carcinoids may occur in association with both MEN type I and MEN type II. MEN should be suspected in patients with foregut carcinoids. Patients with MEN and ectopic ACTH production should be considered to have bronchial carcinoids if they are female and thymic carcinoid if they are male. The thymus should be routinely removed in patients with MEN type I because of the possible presence of an ectopic parathyroid gland in this tissue and to prevent subsequent development of a carcinoid tumor.