Gross anatomical and radioligical techniques have been used to investigate the form and incidence of degenerative joint disease in the knee joint of an inbred strain of mouse (STR/ORT). The disease was seen radiographically as a thickening of the subchondral bone in the medial tibial and femoral condyles predominantly in the males. In fresh and macerated joints only advanced lesions could be seen with light microscopy and these appeared as concave (tibial) or flattened (femoral) erosions with hard shiny eburnated surfaces. Medial patella dislocation was a common feature of the arthrotic joints and there was associated widespread periarticular heterotopic calcification. These growths started as peripheral exostoses from the patella and as small loci of mineralisation within the synovial tissue but later enlarged into plaques of calcified tissue. Ossific centres within the menisci were present in all knees but became greatly enlarged in the osteoarthrotic joints. Calcification of the collateral ligaments occurred in non-arthrotic joints, especially laterally, but progressed to thick ossification in the medial ligament of arthrotic knees. Eventually all these enlarging sites of calcification and ossification began to fuse together, thus limiting the degree of movement of the joint. There was a statistically significant positive relationship between the severe arthropathy and (a) marked patella dislocation and (b) the presence of calcification in the medial collateral ligament.