All 189 knees (157 patients) that had had primary medial compartment arthrosis in 1972 were followed until 1988. After 14 years, tibial osteotomy had been performed in 85 knees and arthroplasty in 33 knees. No major surgery had been performed in 71 knees; of these 71 knees, 31 patients (40 knees) had died. Of the remaining 23 patients (31 knees), the majority had an unsatisfactory knee and managed only on a low activity level. In 20 of 24 knees, progress of the arthrosis was recorded roentgenographically. Because natural course of medial gonarthrosis has a poor prognosis, the majority of patients suffering from this condition will eventually have major knee surgery.