THE circulating mediator (or mediators) responsible for flushing seen in patients with carcinoid syndrome has remained unidentified. The patient described in this study, a 54-year-old woman with a metastatic gastric carcinoid tumor, has experienced approximately four attacks of flushing a day for the past 22 years, each lasting for approximately 30 minutes to three hours.1,2 In 1962 this patient was studied by Oates and Sjoerdsma and found to have markedly elevated levels of urinary free histamine. Administration of L-histidine monohydrochloride markedly increased the frequency, severity and duration of flushes.1,2 Despite this evidence that histamine might participate in the flushing attacks,. © 1979, Massachusetts Medical Society. All rights reserved.