1. A case of essential osteolysis is presented, occurring in a young man of eighteen with no known family history and developing progressively from early childhood. The condition was radiologically evident in the elbows, hands and feet, and was accompanied by atrophy of the cancellous bone of the epiphyses of the shoulders and knees. It was also associated with certain abnormalities of the skull and vertebrae. The patient died from a nephropathy of late onset.

2. Examination of the left foot revealed on the radiologically "lysed" bony extremities a very slow process of erosion affecting essentially the epiphysial and metaphysial cortical bone, of a non-inflammatory nature and accompanied by disappearance of the hyaline cartilage. The extremities not radiologically "lysed" showed signs of erosion that were histologically similar but not macroscopically evident; they were accompanied by regressive changes in the hyaline cartilage.

3. There were no signs of renal osteodystrophy or of Sudeck's dystrophy.

4. Post-mortem tests revealed an increase in the seromucoids and failed to reveal the presence of proline in the serum or of proline and hydroxyproline in the urine.

5. The authors discuss the place of this condition among osteolyses in general.