Background: Antibiotic treatment is commonly associated with gastrointestinal symptoms, in particular diarrhea. The use of lactobacillus preparation has been proposed as a preventive measure. Objectives:We conducted an exploratory study on the effect of Lacteol fort (LF), a heat inactivated lactobacillus preparation, on antibioticassociated bowel disturbances in a primary care population. methods: Consecutive patients attending a primary care clinic because of infection and who were prescribed antibiotics were recruited prospectively. All patients seen by one attending physician were prescribed antibiotics without LF (antibiotic only) (n=96, 29% male, mean age 38 years), while all patients seen by another attending physician were prescribed 2 LF capsules bd for one week during the antibiotic treatment course (antibiotic+LF) (n=88, 43% male, mean age 36.4 years). From the same centre, healthy patients attending the annual health screening (healthy controls) (n=141, 23% male, mean age 39.7 years) were also enrolled in the study. All subjects completed a structured questionnaire at entry, and kept a bowel diary for two weeks from the start of treatment. Results: more patients who received antibiotic treatment reported loose stools ≥ 1 day than healthy subjects who had not received antibiotics, but diarrhea was less in the Lacteol fort treated group (antibiotic only: RR = 1.36, 95% CI 1.07 – 1.72; antibiotic+LF: RR = 1.16, 95% CI 0.89 – 1.51, p=0.046). LF did not reduce the risk of developing bloating, flatus and abdominal pain among patients given antibiotics.Conclusion: Our results suggest that Lacteol fort treatment may reduce the risks of diarrhea associated with antibiotic treatment.