The clinical findings, and urinary and morphological changes in the urinary bladder were investigated in 25 sows with a urinary tract infection. Eubacterium suis was isolated from 12 of the sows but not from the other 13. The clinical signs did not always correlate with the morphological changes. The only clinical sign indicating the beginning of cystitis appeared to be a significant bacteriuria. Other urinary changes occurred later when the inflammatory processes were more severe. In contrast with cystitis due to other bacteria, infection with E suis frequently resulted in a macrohaematuria and urinary pH values above 8.0. However, the light and electron microscopical findings in the bladder mucosa were similar in the sows with and without cystitis due to E suis. The transformation of goblet cells and the development of mucin cysts were probably due to the local bladder defence mechanisms. More severe lesions were observed with E suis infections, which resulted in changes in the ureterovesical junctions and in ascending renal infection and uraemia.