A 2 1/2-year-old Cavalier King Charles Spaniel bitch was examined because of recurring episodes of lameness and joint swelling over a period of 2 years. Clinical examination, radiography, serology, joint-fluid analysis and histopathology of the joint capsule led to a diagnosis of idiopathic, non-infectious, non-erosive arthritis. Combination therapy with prednisolone, cyclophosphamide and azothioprine produced a rapid improvement in the dog's condition.