Three patients with carcinoid tumor associated with hyperparathyroidism are described. All patients showed a high circulating immunoreactive calcitonin level with no differential increase in the neck venous catheterization specimens, suggesting that the high concentrations of circulating immunoreactive calcitonin may have come from the carcinoid tumor. We think that the hyperparathyroidism in these patients was primary and that the association with carcinoid tumor represents another form of multiple endocrine tumor formation. Hyperparathyroidism should be investigated in patients with carcinoid tumor.