Malignant carcinoid tumor altered reactivity of the skin to serotonin in two patients.

Appropriately controlled tests were carried out in two cases of patients with carcinoid tumors and abnormal circulating levels of serotonin to verify a hypothesis that such patients would have a greater tolerance for exogenous serotonin than would normal persons. Contrary to the hypothesis, the patients had not less but more reaction to intradermal injection of serotonin than to histamine and also more reaction than normal persons.If this altered reactivity were to be observed consistently in large series of patients with malignant carcinoid disease, it might be used as a presumptive test.