1. A case of essential osteolysis, progressing for thirteen years and involving the right femur and pelvis of a boy who was thirteen years old at the time of onset, is presented.

2. The histological appearance of fragments of bone and soft tissue removed from the affected thigh is described.

3. No evidence that might throw light on the etiology of the disease was obtained from biochemical investigations or from the histological studies.

4. None of the many lines of treatment that were tried had any effect upon the progress of the disease.