In order to assess the potential of lactobacilli to cause serious infections, we studied the prevalence of bacteremia due to Lactobacillus species during a 4-year period (1989-1992) in southern Finland, which has a population of about 2.5 million. Among 3,317 blood culture isolates, lactobacilli were identified in eight patients, five of whom had a severe disease predisposing to bacteremic complications. The eight strains isolated were identified to the species level and typed by carbohydrate fermentation tests and by direct sequencing of enzymatically amplified 16S rRNA. The results did not provide evidence that any particular species or subspecies of Lactobacillus was the cause of the infections; no infections caused by isolates similar to the recently introduced dairy probiotic strain, Lactobacillus GG (ATCC 53103), were observed. The data show an infrequent association of lactobacilli with bacteremic infections in spite of the ubiquitous presence of these organisms in the gastrointestinal tract and their widespread consumption in fermented milks; thus, there is strong evidence that their pathogenic potential is very low.