Leuconostocs, members of the family Streptococcacae, have only recently been recognized as potential pathogens. We describe six cases of leuconostoc bacteremia and review 11 additional cases of infection reported in the literature. Fifteen patients with bacteremia ranged from neonates to persons aged 78 years. Almost all were hospitalized with significant underlying diseases, had received previous antibiotic therapy, and had undergone procedures that interrupted the normal integumentary defense. Leuconostoc bacteremia was associated with fever, leukocytosis, and gastrointestinal complaints. Eight of 15 patients had polymicrobial bacteremia, seven of these eight with staphylococcal species. Clinical isolates of Leuconostoc were frequently misidentified, usually as viridans streptococci. All clinical isolates identified to date--and most agricultural isolates--demonstrate a high level of resistance to vancomycin. Successful regimens for treatment of Leuconostoc include high-dose penicillin, clindamycin, and where appropriate, removal of infected intravascular catheters. Susceptibility testing of all gram-positive bacteria isolated from normally sterile body sites is recommended.