AbstractBackgroundFollicular cystitis is an uncommon inflammatory change in the urinary bladder wall characterized by the formation of tertiary lymphoid structures (TLSs) in the submucosa.ObjectivesTo characterize clinical and pathologic features of follicular cystitis in dogs and to explore in situ distribution and possible role ofEscherichia colias an associated cause.AnimalsEight dogs diagnosed with follicular cystitis and 2 control dogs.MethodsRetrospective descriptive study. Dogs diagnosed with follicular cystitis (macroscopic follicular lesions in the urinary bladder mucosa and histopathologic detection of TLSs in bladder wall biopsies) were identified from medical records. Paraffin embedded bladder wall biopsies were subject to in situ hybridization forE. coli16SrRNA identification.ResultsFollicular cystitis was diagnosed in large breed (median weight 24.9 kg, interquartile range [IQR] 18.8‐35.4 kg) female dogs with a history of chronic recurrent urinary tract infections (UTIs; median duration of clinical signs 7 months, IQR 3‐17 months; median number of previous UTIs 5, IQR 4‐6). PositiveE. coli16SrRNA signal was detected within developing, immature and mature TLSs in 7/8 dogs, through submucosal stroma in 8/8 dogs and within the urothelium in 3/8 dogs.Conclusions and Clinical ImportanceChronic inflammation associated with an intramuralE. coliinfection in the urinary bladder wall represents a possible triggering factor for the development of follicular cystitis.