Therapy with metronidazole is the recommended option in giardiasis. However, some clinical trial reports suggest the appearance of drug resistance to explain therapeutic failure. Several investigations have been carried out on the effect of probiotic microorganisms for preventing or treating gastrointestinal diseases, but little is known about their efficacy against protozoal infections. The principal objective of our study was to evaluate the efficacy of Saccharomyces boulardii against Giardia lamblia infections. A double-blind, placebo-controlled study was carried out on adult patients with giardiasis. Group 1 (30 patients) included metronidazole 750 mg 3 times daily along with S. boulardii capsules (250 mg b.i.d. orally) for 10 d while group 2 (35 patients) was treated with metronidazole 750 mg 3 times daily and with empty capsules as placebo for 10 d. Patients were re-examined at 2 and 4 weeks after treatment, and stool examinations were performed. At week 2, G. lamblia cysts were detected in 6 cases (17.1%) of group 2 and none in group 1. At the end of the fourth week, presence of the cysts continued in the same 6 cases in group 2 (control group). These findings indicated that S. boulardii may be effective in treating giardiasis when combined with metronidazole therapy.