The correlation between arthroscopic and radiographic osteoarthrosis was studied in 63 patients (51 +/- 12 years) with degenerative changes in the medial compartment of the knee involving the medial meniscus with or without chondral damage. Chondral damage on arthroscopy was classified in grades 1-3 modified after Outerbridge. The radiographic evaluation was done according to Ahlbäck classification. Twenty-eight patients (44%) did not show any significant chondral damage on arthroscopy (grades 0-1). The radiographic findings in this group were usually normal. Three patients, however, showed a slight narrowing of the medial joint space (Ahlbäck I). All three had severely degenerated menisci, and two of them showed superficial fibrillation of both the tibia and the femur. Twenty-four patients (38%) showed a partial thickness chondral damage with fragmentation of the surface (grade 2). Radiographic findings were usually normal; however, patients with involvement of both the tibia and the femur often showed a joint space narrowing on radiographs. Eleven patients (18%) showed destruction of the cartilage down to subchondral bone (grade 3). The typical radiographic finding was a joint space obliteration (Ahlbäck II). Involvement also of the lateral compartment as assessed on arthroscopy was significantly more common in patients with 2nd and 3rd degree chondral disease and Ahlbäck II and III on radiographs than in those with 2nd and 3rd degree chondral damage and normal radiographs. In all but one of the patients with involvement of both compartments, the lateral compartment was considered normal on radiographic examination.