The determination of the urinary excretion of 5-hydroxyindoleacetic acid (5-HIAA), the major metabolite of 5-hydroxytryptamine (Serotonin), is part of routine laboratory diagnosis of carcinoid tumours. Carcinoid may develop even in the terrain of Crohn’s disease and should be excluded particularly in torpid forms and in those with insufficient response to treatment. The basic drug in Crohn’s disease are aminosalicylates (mesalazine-Pentase). 15 patients withCrohn’s disease treated with aminosalicylatesexhibited regular massive interferencewhen determining 5-HIAA in urine using the HPLC method with electrochemical detection. The elution time of interfering peak was identical with that of 5-HIAA standard. Falsely increased levels of 5-HIAA excretion ranged between 268 and 10250 µmol/24 h mean 3327 µmol/24 h. The repeated determination of the urinary excretion of 5-HIAA in the followed-up patients after 5-days’ discontinuation of aminosalicylates a level exceeding the upper limit of the reference range was not recorded in any patient. Conclusion: possible interference must be always eliminated if a positive finding of increased excretion of 5-HIAA in urine is recorded. Besides the possible influence of intake of food with a high content of hydroxyindole, possible drug interference must be considered, and the examination should be repeated after discontinuation of the suspicious drug. If aminosalicylates are administered, the treatment must be discontinued at least 4 days before collecting urine for the determination of 5-HIAA.

        Key words: 5-HIAA, HPLC method, interference, aminosalicylates, Crohn’s disease, carcinoid.