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A 10 wk old female border collie was presented for hemorrhagic diarrhea and pelvic limb lameness. Examination revealed pain and effusion in multiple appendicular joints and pyrexia. Clinicopathologic testing revealed moderate neutropenia as well as nondegenerate neutrophilic inflammation in multiple joints. Radiographs showed capsular joint swelling and heterogeneous metaphyseal lucencies in the distal radius, ulna, femur, and tibia. Genetic testing confirmed a mutation in the vacuolar protein sorting-associated protein 13B gene and a diagnosis of trapped neutrophil syndrome (TNS). Within 24 hr of initiating prednisone therapy (1 mg/kg, per os , q 12 hr), the dog was afebrile and nonpainful with normal ambulation. Lameness recurred twice over the next 5 mo. At 9 mo of age, diagnostics showed severe erosive polyarthritis of both stifles with an inflammatory leukogram and arthrocentesis findings consistent with septic arthritis, and the dog died despite antibiotic therapy. This is the first case of TNS described in the North American literature, and it is unique in that we had the opportunity to document progression of radiographic abnormalities over more than 6 mo. TNS should be considered in young border collies with signs suggestive of immune-mediated polyarthritis, septic arthritis, or hypertrophic osteodystrophy, combined with neutropenia or gastrointestinal signs.