1. Two patients are reported with progressive fibrosis of the vastus intermedius muscle causing limitation of flexion of the knee. The first patient, a girl of ten years who was one of binovular twins, noticed difficulty in walking at the age of five, the disability increasing with age. The other girl, aged three, was one of uniovular twins and displayed the abnormality as soon as she started walking.

2. The nature of the condition remains obscure.

3. Surgery is effective, and simple division of the affected muscle segment is recommended. A good range of passive flexion is obtained at once; the active range can be obtained thereafter by physiotherapy. Because this requires the patient's cooperation it is suggested that the best age for surgery is about five or six years.