The goal of the present study was to describe the clinical, haematological and ultrasonographic findings and treatment of 17 cattle with pyelonephritis. Fifteen cattle had an abnormal general condition, which varied in severity; five animals had signs of colic. The urine was brownish-red in 11 animals and cloudy in 13. Clumps of purulent material were seen in the urine of nine animals and clots of blood in two. The specific gravity was lower than normal in 13 animals and ranged from 1.005 to 1.020. A urine test strip revealed protein in 16 animals, blood in 16 and leukocytes in 12. Bacteriological examination of urine yielded Corynebacterium renale in 11 animals, Arcanobacter pyogenes in two and Escherichia coli in one. Rectal examination revealed abnormalities of the urinary tract in 11 animals; there was dilatation of the left ureter and/or enlargement of the left kidney in eight cases, and dilatation of the right ureter and/or enlargement of the right kidney in three others. The most frequent abnormal haematological finding was an increase in the serum concentrations of total protein, fibrinogen, urea and creatinine, a decreased haematocrit and a positive glutaraldehyde test. In 13 animals, ultrasonography via the rectum and right flank using a 5.0MHz transducer revealed dilatation of the right or left ureter, cystic lesions in one or both kidneys and dilatation of the renal sinus. Eight animals were euthanased or slaughtered at the owners' request or because of a poor prognosis. Nine (53%) animals were successfully treated; five received antibiotics and four underwent unilateral nephrectomy and antibiotic therapy. The treated animals were clinically healthy when discharged from the clinic 10-21 days after admission. A follow-up via telephone 8-24 months later revealed that none had experienced complications and all were in full production. In cattle with severe unilateral pyelonephritis, unilateral nephrectomy is the treatment of choice.