We have assessed 270 consecutive patients (age range 0.8-20 years) referred for endoscopic study because of abdominal pain during 32 months. Helicobacter pylori (HP) was detected by culture in 91 cases (33.7%). HP colonization increased significantly with age (p less than 0.01). Nine patients less than 5 years of age were colonized by HP. A previous history of peptic ulcer disease in first-degree relatives was significantly more frequent in the HP-positive group (p less than 0.001). The frequency of HP positiveness as related to diagnosis was: normal, 3.3%; nonactive chronic gastritis, 100%; active chronic gastritis, 97.2%; gastric ulcer, 75%; and duodenal ulcer, 90.9% (p less than 0.001). Endoscopic nodular antritis was a frequent (67%) and specific finding; this presence was associated with that of lymphoid follicles in the histopathological study. Signs of histological activity were observed in 55.9% of the HP-positive patients. The histological colonization by HP was assessed semiquantitatively, and a significantly greater HP colonization score was observed in patients with signs of histological activity (p less than 0.001). A significant correlation was found between HP colonization score and histological score (rs = 0.574), with a significant association between the degree of HP colonization and the histologic categories (p less than 0.001). The present study suggests a pathogenic role of HP in the development of gastroduodenal disease in children.