At operation for the correction of cubitus varus by removal of a wedge based laterally, the radial nerve fortunately was first explored and found to run through a transverse bony tunnel at the posterior level of a supracondylar fracture sustained eight years previously. After elevation of the bony tunnel and nerve en bloc the osteotomy was completed; neurapraxia of the radial nerve soon recovered. With hindsight, the entrance and exit foramina of the tunnel could be clearly seen in the antero-posterior radiograph taken before the operation.

1. Two cases of swelling in the popliteal fossa due to a local hypertrophy of the semimembranosus muscle are described and two further cases thought to be of similar nature are noted.

2. The swelling was soft, not tender, fairly well circumscribed, and without signs of inflammation. Its most characteristic feature was an increase in size and hardening of the swelling during active flexion of the knee.

3. This condition should be considered in differential diagnosis of swellings of the popliteal fossa.