Synovial membranes from 54 dogs were collected at the time of surgery to stabilize the stifle joint following rupture of the cranial cruciate ligament. Histological evaluation revealed lymphoplasmacytic synovitis with the formation of distinct, nodular aggregates in 36 synovial membranes. The remainder had evidence of chronic synovitis with variable numbers of diffuse, mononuclear cells. The presence of a potentially immune-mediated synovitis in the majority of cases suggests the need for further study.