OBJECTIVE: To determine effects of cranberry extract on development of urinary tract infection (UTI) in dogs and on adherence of Escherichia coli to Madin-Darby canine kidney (MDCK) cells. ANIMALS: 12 client-owned dogs (in vivo experiment) and 6 client-owned dogs (in vitro experiment). PROCEDURES: 12 dogs with a history of recurrent UTI received an antimicrobial (n=6) or cranberry extract (6) orally for 6 months. Dogs were monitored for a UTI. For the in vitro experiment, cranberry extract was orally administered to 6 dogs for 60 days. Voided urine samples were collected from each dog before and 30 and 60 days after onset of extract administration. Urine was evaluated by use of a bacteriostasis assay. An antiadhesion assay and microscopic examination were used to determine inhibition of bacterial adherence to MDCK cells. RESULTS: None of the 12 dogs developed a UTI. The bacteriostasis assay revealed no zone of inhibition for any urine samples. Bacterial adhesion was significantly reduced after culture with urine samples obtained at 30 and 60 days, compared with results for urine samples obtained before extract administration. Microscopic examination revealed that bacterial adherence to MDCK cells was significantly reduced after culture with urine samples obtained at 30 and 60 days, compared with results after culture with urine samples obtained before extract administration. CONCLUSIONS AND CLINICAL RELEVANCE: Oral administration of cranberry extract prevented development of a UTI and prevented E. coli adherence to MDCK cells, which may indicate it has benefit for preventing UTIs in dogs.