To gain insight into the mechanisms by which Lactobacillus blocks the adherence of uropathogens to vaginal epithelial cells and inhibits their growth, 15 Lactobacillus strains and 22 uropathogens were studied. Lactobacilli from hemagglutination group III, identified as Lactobacillus crispatus, showed greater capacity to block uropathogen adherence than those from hemagglutination groups II and I (61.9%, 49.5%, and 52.6% of blockage, respectively). Pseudomonas aeruginosa PA5 and Klebsiella pneumoniae KP7 were the uropathogens most susceptible to blockage, and Staphylococcus aureus SA11 and Proteus mirabilis PM1 were the most resistant. Lactobacillus inhibited uropathogen growth better in liquid assays; the 3 Lactobacillus groups showed similar inhibitory power (72.3%, 71.9%, and 74.2% of light transmittance). P. aeruginosa PA5 was the most inhibited, and Enterococcus species E15 was the least inhibited. There is considerable variation among Lactobacillus strains regarding their adherence to uroepithelium, blockage of uropathogen attachment, and inhibition of uropathogen growth. Although these properties are independent, they may coincide and therefore allow for these strains to balance the vaginal ecosystem and to make them useful as probiotics.