Saccharomyces boulardii (S.b.) is used for the prevention and treatment of diarrhea of different etiologies. We prospectively investigated the effects of S.b. on lymphocytes and duodenal mucosa. Before and after oral administration of S.b. for 3 weeks, circulating and intestinal lymphocytes were isolated and characterized by flow cytometry. Trophic effects on duodenal mucosa were investigated by morphometry and determination of brush border enzyme activity. Results were compared intraindividually before and after S.b. In intestinal lymphocytes no phenotypic changes were observed. CD4+ cells of the peripheral blood had a significantly increased expression of CD25 (p < 0.02). None of twelve volunteers had an increase in villous surface area (n.s.). Immunoglobulin A content in small intestine secretion was unaltered. An increase in brush border enzyme activity of lactase, α-glucosidase, and alkaline phosphatase was observed (p < 0.01). Our findings indicate that S.b. has a positive effect on the maturation of enterocytes and only a minor influence on lymphocytes