1. Twelve children are described in whom gradual limitation of knee flexion developed, without previous injury or inflammation.

2. Clinical and histological investigation showed progressive fibrous degeneration of the vastus intermedius and rectus femoris.

3. As the patella showed signs of retarded development a particular form of myodysplasia, probably congenital, may be supposed.

4. Conservative treatment always failed to stop the progress of this disease.

5. Surgical treatment (Bennett's method or an elongation of the ligamental apparatus of the intermedius muscle) was carried out in ten cases and gave good permanent results. Flexion remained possible only to the extent obtained at operation; rehabilitation gave no further improvement.

6. In the opinion of the author, these cases occur more often than is generally believed, but they are recorded under a different diagnosis and consequently are not submitted to rational treatment.