1. French in 1946 presented eight cases of arachnoiditis complicating a lumbar disc lesion; five further cases are reported here.

2. It is suggested that repeated minor persistent trauma produces the strictly localised changes in the arachnoid, in the same sort of way that it may produce ulnar neuritis at the elbow.

3. The various causes of arachnoiditis are discussed.

4. The place of operation has been found difficult to assess.