The aim of this study was to determine the lactose absorption capacity and possible existence of bacterial overgrowth in the small bowel in asymptomatic school children of low social economic level in Marilia, a city located in the interior of São Paulo state. Eighty three children aging 7 to 15 years old without any gastrointestinal manifestations at least 30 days prior to the tests were studied. All the patients had fasted for at least 8 hours before the tests were performed. Lactose absorption was evaluated by breath hidrogen test after an overload of lactose 18 g in 10% aquous solution. Lactose intolerance was determined by the occurrence of clinical symptoms, such as diarrhea, abdominal pain, flatulence, etc in the following 24 hours after the test was performed. Bacterial overgrowth was evaluated by the breath hidrogen test after a 10 g lactulose load in aqueous solution. Lactose malabsorption was detected in 19 (22.9%) children and lactose intolerance was observed in 10 (12%) children. Lactose intolerance was more frequently observed in children who showed lactose malabsorption (6/19; 31.6%) than in those who presented a normal test (4/64; 6.3%) (P = 0.008). Bacterial overgrowth was detected in six (7.2%) children and showed no statistical relationship with lactose malabsorption. Ontogenetic lactose malabsorption verified in this group of school children is similar to the reported for Caucasian populations. Presence of bacterial overgrowth confirms the existence of asymptomatic environmental enteropathy in children of low social economic level.