BACKGROUND Chronic idiopathic urticaria (CIU) is considered a complex and multifactorial disease. Excessive histamine intake may induce an attack of urticaria. The main enzyme for histamine metabolism is diamine oxidase (DAO). OBJECTIVE Plasma histamine concentrations and DAO activities were evaluated to determine whether there are abnormalities in the histamine metabolism of CIU patients. METHODS Seventy-five CIU patients and twenty-five healthy control subjects were included in the study. Blood was taken from all subjects to measure plasma levels of the histamine and DAO. RESULTS Mean plasma histamine levels were significantly higher in CIU patients (11.59±10.98 nM) than in the control subjects (8.75±2.55 nM) (p=0.04). Mean DAO activities were lower in patients of CIU (80.86±26.81 histamine degrading unit [HDU]/ml) than in the controls (81.60±9.67 HDU/ml), but without significant difference. In 15 CIU patients with gastrointestinal symptoms, the mean histamine concentration was higher (12.43±7.97 nM) and DAO activity was lower (77.93±27.53 HDU/ml) than in the remaining 60 CIU patients without gastrointestinal symptoms (11.38±11.67 nM and 81.58±26.82 HDU/ml), without significant difference. The relationship between DAO activity and plasma histamine concentrations showed a significant negative linear value (p=0.001). There were no significant relationships between plasma histamine concentrations and symptom severity score. CONCLUSIONS In CIU patients, a high plasma histamine concentration may not be explained by DAO activity. CIU patients with gastrointestinal (GI) symptoms showed no significantly lower DAO activity. Larger group studies are required to elucidate the relationship between plasma histamine concentrations and DAO activity, especially of CIU patients with GI symptomsto understand the difference in CIU patients with and without GI symptoms.