Probiotic strains of lactobacilli are increasingly being used in clinical practice because of their many health benefits. Infections associated with probiotic strains of lactobacilli are extremely rare. We describe 2 patients who received probiotic lactobacilli and subsequently developed bacteremia and sepsis attributable to Lactobacillus species. Molecular DNA fingerprinting analysis showed that the Lactobacillus strain isolated from blood samples was indistinguishable from the probiotic strain ingested by the patients. This report indicates, for the first time, that invasive disease can be associated with probiotic lactobacilli. This report should not discourage the appropriate use of Lactobacillus or other probiotic agents but should serve as a reminder that these agents can cause invasive disease in certain populations.