Urticaria and angioedema symptoms result primarily from the physiological actions of histamine. Some individuals with urticaria have a decreased ability to degrade dietary histamine before it enters the circulation. Foods high in histamine, such as fermented foods, may exacerbate urticaria and angioedema in these individuals. Certain food additives may increase endogenous release of histamine and urticaria and angioedema symptoms. The objective of this study was to evaluate the effect of a histamine-reducing diet on urticaria and angioedema symptoms, and on nutrient intake. Nineteen subjects with chronic urticaria or angioedema were randomized to a treatment group (n=9) or a control group (n=10). The treatment group followed a histamine-reducing diet, and the control group eliminated artificial sweeteners from their diets. The subjects recorded antihistamine medication intake, number of wheals, the severity of pruritus and the severity of angioedema for two weeks before starting the diet and for six weeks during the dietary intervention. Subjects completed three-day food records every two weeks. There was a marginally significant decrease in the number of antihistamine tablets taken in the histamine-reducing diet group compared with the control group, and two of nine treatment subjects had dramatically improved symptoms. During the study there was no significant risk of nutritional deficiency for either group.