Objective To attempt to determine whether a relationship exists between carcinoid tumors and sarcoidosis. Material and Methods We present a series of seven case reports and discuss hypotheses about possible disease associations. Results Certain malignant lesions have tended to occur in patients with sarcoidosis. Seven patients who were encountered at Mayo Clinic Rochester between 1950 and 1994 had both sarcoidosis and carcinoid tumors. These patients ranged in age from 31 to 66 years, and three of the patients had a history of benign thyroid disorders. Malignant tumors have been thought to be related to sarcoidosis in one of two ways: (1) immunologic abnormalities in sarcoidosis may promote the development of neoplasms or (2) malignant disease may promote the onset of sarcoidosis either by causing local sarcoid reactions that progress or by directly initiating the manifestations of systemic sarcoidosis. Because the chronology of events differed in our seven cases, various mechanisms of action may have a role in the manifestations of these two disease entities. Our cases emphasize the importance of avoiding the diagnosis of disseminated malignant disease in patients with cancer and associated hilar and mediastinal lymphadenopathy without biopsy confirmation of metastatic disease. Conclusion Application of the knowledge gained about the mechanisms of disease in sarcoidosis will perhaps facilitate identification of the pathogenesis of carcinoid tumors and other neuroendocrine tumors.