Synovial hemangioma in a dog. 
To report arthroscopic diagnosis and treatment of synovial hemangioma in a dog.  Case report.  Standard Poodle (8-year-old neutered male).  A soft tissue density mass observed radiographically in the left stifle joint of a dog with a 2-month history of recurrent lameness, hemarthrosis, and a slight cranial drawer sign, was located by diagnostic arthroscopy and surgically excised via arthrotomy.  Histologic evaluation revealed tissue composed of variably sized cavernous vascular spaces filled with erythrocytes that were considered compatible with synovial hemangioma.  In this dog, synovial hemangioma evident as a soft tissue mass on radiographs was associated with chronic lameness and hemarthrosis, and resolved with surgical excision.  Synovial hemangioma, although seemingly rare in dogs, should be considered in the differential diagnosis for nontraumatic, recurrent lameness, and unresponsive to anti-inflammatory therapy when there is a circumscribed intracapsular soft tissue mass evident radiographically together with hemarthrosis.