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-1 ml-1; 2) gastroenteritis (acute phase) (n = 11), 31 +/- 4.9 pmol h-1 ml-1; 3) gastroenteritis (healing phase) (n = 21), 18 +/- 1.9 pmol h-1 ml-1. Patients with gastroenteritis in the healing phase had significantly lower DAO values when compared to control patients (p less than 0.001) and to gastroenteritis patients in the acute phase (p less than 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 less than 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.