Colonization of the gut by intestinal bacteria begins at birth and progresses rapidly in the immediate postnatal period. Host defense mechanisms that mediate enteric colonization include gastric acidity and intestinal motility. The small bowell overgrowth syndrome is a condition characterized by large numbers of bacteria, often anaerobes, in the upper intestine. Steatorrea, carbohydrate malabsorption and abdominal pain are frequently present. Predisposing conditions are localized anatomic disorders (surgical blind loops, small bowel strictures caused by surgery or Crohn's disease, short-gut syndrome without ileocaecal valve), motility derangements or reduction of gastric acidity. Diagnosis of the overgrowth syndrome is often difficult and quantitative cultures of jejunal-aspirated fluid is the best diagnostic test. Antimicrobial therapy directed against anaerobes is often successful, but the best therapeutic approach is the correction of predisposing conditions, if present.