The purpose of this study was to investigate the prevalence and meaning of small intestine bacterial overgrowth (SIBO) in patients with chronic diarrhea and normal control subjects.To that effect, a hydrogen breath test was performed using lactulose on 54 patients with chronic diarrhea and 16 normal control subjects. The presence of SIBO was defined as an increase in the hydrogen concentration, of > 20 parts per million above the basal value in any of the breath tests after the ingestion of lactulose. In addition, the logarithms of the areas under the hydrogen concentration curves were calculated.SIBO was present in two (12.5%) of the 16 normal control subjects. The frequency of SIBO was higher in patients with chronic diarrhea (24.1%) than in the normal control subjects; however, this difference was not statistically significant (p:0.27). Finally, the mean + sd of the logarithms of the areas under the hydrogen concentration curves was significantly higher in patients with chronic diarrhea (6.485 + 0.751) than in normal control subjects (6.135 + 0.500) (p<0.05).The results obtained show that SIBO occurs in a certain percentage of the population, without presenting diarrhea. For SIBO to produce diarrhea, it probably requires some additional characteristics. However, it is worth it to suspect and investigate for SIBO in all chronic diarrhea cases.