It is frequently assumed that persisting instability of the knee joint leads to gonarthrosis. We have reviewed recent and primary radiograms of 32 patients with old knee trauma resulting in prolonged capsular and ligamentous instability in order to study the eventual development of secondary gonarthrosis. Our results were as follows: All primarily intact joints developed gonarthrosis after an average of 46 months of permanent instability. Primarily existing gonarthrosis increased with exception of 3 cases. The tendency to develop an arthrosis seems to be proportional to the duration of instability; the type of instability apparently has no influence on the development of an instability gonarthrosis. Operative procedures having been performed prior to final stabilysing surgery obviously do not cause the development of secondary gonarthrosis. The persisting instability of the knee joint after combined discontinuation of capsule and ligaments therefore may be regarded as a prearthrotic factor.