Small bowel bacterial overgrowth and related diarrhea is a condition that frequently accompanies anatomic disorders, surgically created blind loops or strictures with partial small bowel obstruction and although it is often controlled with antimicrobial therapy, alternative treatment may be needed. The aim of this study was to evaluate the efficacy of an oral probiotic preparation of 2 viable lyophilized strains of lactobacilli (1.5 g each) compared with placebo. Twenty two patients with proven overgrowth and chronic diarrhea are described. In random order and double-blind fashion, 2 groups of patients received identical capsules with both Lactobacillus casei and L. acidophillus strains CERELA (12 patients) (LC) and placebo (10 patients) (P) during three consecutive periods of 7 days each followed by a similar three periods of control after withdrawal. At the end of each period the mean daily number of stools, glucose breath H2 test, and symptoms were considered. Lactobacillus were investigated in feces in both groups at day 0 (baseline), on day 21 of treatment with LC and P and on day 21 after withdrawal. Compared with P a significant reduction in mean daily number of stools was achieved with LC (p < 0.005) at 15 days, and (p < 0.0005) at 21 days and the effect was sustained at 7 days and 15 days (p < 0.005) after withdrawal. With respect to breath H2 level a significant decrease in H2 concentration was noted at 7 days (p < 0.005) at 15 days, and 21 days (p < 0.0001) with LC and only a significant decrease (p < 0.005) was observed at 7 days after withdrawal. No significant changes were observed with respect to symptoms. The Lactobacillus CERELA strains were isolated from the feces in all patients LC (n = 12) on day 21, and by contrast no Lactobacillus were observed except in two patients out of seven patients after withdrawal. In summary, this study provides evidence that LC are effective for treatment of bacterial overgrowth--related chronic diarrhea, and suggest that probiotics must be used with continuity.