Serum diamine oxidase (DAO) activities were measured in 20 control patients and in 24 patients with gastroenteritis. Results (mean ± SE) were as follows: 1) control patients (n=20), 36.2 ± 5.7 pmol h-1ml-1; 2) gastroenteritis (acute phase) (n=11), 31 ± 4.9 pmol h-1ml-1; 3) gastroenteritis (healing phase) (n=21), 18 ± 1.9 pmol h-1ml-1. Patients with gastroenteritis in the healing phase had significantly lower DAO values when compared to control patients (p<0.001) and to gastroenteritis patients in the acute phase (p<0.05). In eight patients where both acute and healing phase values could be measured, a significant decrease between acute and healing phase was found (p<0.001). Patients with severe gastroenteritis tended to have lower DAO activity values than patients with moderate gastroenteritis (p=0.10). Our results support the hypothesis that serum DAO activity is a marker of the total mass of functional enterocytes, the decrease of which during gastroenteritis is reflected in a decrease of serum DAO activity values.