Oral administration of probiotic compounds has been demonstrated to be well tolerated and proven to be safe in 143 human clinical trials occurring between 1961 and 1999. No adverse effects or events were reported in any of the 7526 subjects participating in these trials. However, rare cases of local or systemic infections, including septicemia and endocarditis due to Lactobacillus, have been reported. These infections have occurred in immunocompromised patients with aplasia, organ transplantation and human immunodeficiency virus infection. In most of these cases, the source of the infection was the commensal Lactobacillus flora, rather than an ingested bacteria supplement, suggesting that these bacteria can act as opportunistic pathogens. With regard to Saccharomyces infections, there have been few reports of fungemia due to Saccharomyces species, again, usually in immunocompromised patients receiving high enteral doses of Ultra-Levure (Biocodex, Montrouge, France) containing S boulardii (1.5 g/day). Although rare, these reports suggest that caution and further studies are necessary to assess the safety of probiotic bacteria for immunodeficient hosts.