The exact etiology of carcinoid flushing remains unknown, but the symptoms are probably mediated through release of one or several humoral substances. Flushing seen in fore-gut carcinoids (gastric carcinoids) has been ascribed to excessive histamine release, whereas flushing seen in mid-gut carcinoids (ileal carcinoids) tentatively has been ascribed to excessive release of serotonin, bradykinin, substance P, substance K or eledoisin. In this study plasma histamine was measured in 8 patients with mid-gut carcinoids and carcinoid syndrome using an enzymatic isotopic method in order to evaluate histamine as the vasoactive agent in patients with ileal carcinoid tumours and carcinoid syndrome. All patients had raised plasma histamine values. In patients with mid-gut carcinoids histamine may be one of the substances mediating flushing.