Group B Streptococcus (GBS) causes urinary tract infections, but the pathogenic mechanisms underlying GBS urinary tract infections are unknown. We investigated whether uropathogenic GBS can bind to bladder uroepithelium to initiate urinary tract infection. Uropathogenic GBS isolated from a patient with acute cystitis bound to human T24 bladder myoepithelial cells in close association with F-actin in statistically significantly higher numbers compared with nonuropathogenic GBS. In vivo modeling using transurethrally infected mice revealed superior fitness of uropathogenic GBS for bladder colonization and potent uropathogenic GBS-specific up-regulation of interleukin la during infection. Thus, binding of uropathogenic GBS to uroepithelium and vigorous induction of interleukin la represents the initial stages of GBS urinary tract infection. © 2010 by the Infectious Diseases Society of America. All rights reserved.