The aerobic bacteria microflora of the upper small bowel in normal children and in children with acute diarrhea was studied. Forty one children aging between 2 months and 5 years were divided in 3 groups: Group I--13 wellnourished children with no signs of gastrointestinal abnormalities remained intubated for less than 4 hours; Group II--11 wellnourished children that remained intubated for a period of time varying from 4 to 24 hours; Group III--17 children with acute diarrhea. There was a striking difference in the bacterial pattern between children from groups I and II and these findings were attributed to the prolonged time of intubation. In group I the jejunal fluid was sterile in approximately 40% of the children and in the others there was only growth of a GRAM positive flora while in group II the jejunal fluid never presented as a sterile one, and in approximately 73% of the cases there was a growth of a mixed GRAM positive and GRAM negative microflora, mainly due to Enterobacteria. In group III an aerobic microflora different from that one verified in group I was found in 76,4% of the patients. In 47.05% of the cases a bacterial growth equivalent to the fecal flora has been observed, in 5.89% Pseudomonas aeruginosa was located and in 23.52% yeasts were cultured from the jejunal fluid. These data confirm the hypothesis that bacterial and fungi infections are most prevalent in underdeveloped countries. This is probably due to the poor sanitary conditions observed in these countries associated with a high index of environmental contamination.