The aim of this study was to examine whether a vaginal Escherichia coli colonization, mimicking the one seen in UTI-prone females, could be induced by local cephadroxil administration. When five adult cynomolgus monkeys were given a vaginal flush with a P-fimbriated E. coli strain, none became persistently colonized. When such colonization attempts were preceded by cephadroxil administration a persistent colonization occurred in 9/10 experiments. Cephadroxil also promoted a spread of fecal E. coli strains to the vagina. Reduction of the anaerobic vaginal flora can explain the breakdown of the colonization resistance. Clinical observations suggest that accumulation of E. coli around the urethral orifice increases the risk of UTI. Therefore antibiotics which promote such colonization may increase the risk for UTI in susceptible patients. From this point of view antibiotics such as cephadroxil may be less suitable for treatment of UTI-susceptible patients.