Diagnosis of histamine intolerance (HIT) has been based on low serum diamine oxidase (DAO) values, functional gastrointestinal disorders and improvement of symptoms with a histamine-reduced diet (HRD). In a retrospective analysis of outpatients' charts we identified 101 patients with HIT. After a median of 13 months, a questionnaire was distributed to the patients so that they could be classified into four diet-compliance groups. Calculated with all 101 patients we found an increase of serum DAO values due to a HRD. In the 63 patients that completed the questionnaire, we found that 50 patients had improvement of symptoms or no continuing symptoms. A significant increase of serum DAO levels was found in the patients with strict and occasional diet compliance. Therefore, we demonstrate that a HRD is not only improving symptoms in HIT, but is causing an increase in serum DAO values that correlates with the degree of diet compliance.