During a period of 30 months, cow's milk- or soy protein-induced colitis was diagnosed in 22 infants. All patients presented with hematochezia. Only three patients had substantial diarrhea. Colonoscopic evaluations demonstrated a characteristic mucosal appearance, manifested by aphthoid ulcerations with intervening, normal-appearing mucosa. Histologic findings included surface ulcerations and associated inflammation of the lamina propria and submucosa, marked by substantial eosinophilic infiltration. Both clinical symptoms and colonoscopic abnormalities rapidly resolved in all patients after the elimination of cow's milk and/or soy protein from the diet. These data indicate that hematochezia may be the predominant symptom of cow's milk protein and soy protein intolerance in many affected infants, and that colonoscopy is the most effective means of establishing a definitive diagnosis in this patient population.