Objective-To determine the clinical and pathologic findings in dogs with primary bicipital tenosynovitis. Animals-19 dogs with 20 shoulder joints treated surgically for bicipital tenosynovitis and 8 shoulder joints from 4 clinically normal dogs. Procedure-Histologic abnormalities of tendon sheaths of the biceps brachii in affected dogs were determined by use of comparison with findings in clinically normal dogs. Specimens were graded for inflammation, fibrosis, villous hypertrophy, vascular prominence, and synovial cell proliferation. Histopathologic results were statistically evaluated for relationship with clinical findings and treatment before surgery. Results-Synovial villous hypertrophy and vascular prominence were the most consistent histologic findings in 16 and 14 of 20 affected joints, respectively. Evidence of inflammation was lacking in 6 joints. Ten joints had inflammatory cell infiltration of the tendon sheath. Plasma cells and lymphocytes were the most common infiltrates; however, the type and amount of inflammatory cell infiltrate were variable. Fibrosis of the tendon sheath was seen in 8 joints, and synovial cell proliferation was seen in 11 joints. Other changes included accumulation of hemosiderin, focal calcification, osseous metaplasia, lysis of collagen, and fibrocartilaginous metaplasia. No significant relationship was detected between histopathologic findings and clinical findings or treatment before surgery. Conclusions and Clinical Relevance-inflammation was more variable than hypothesized and may not be a consistent pathophysiologic feature of bicipital tenosynovitis. In some dogs, this disease may be the result of a degenerative process rather than an inflammatory process.