1. A case of solitary bone cyst of the humerus of thirty-eight years' standing is reported.
2. It had a fibrous lining which was extensively calcified in almost its entire extent and which cast a shadow in the x-ray.
3. The fluid within the cyst was loaded with cholesterol crystals. In the bone surrounding the cyst lining there was no fibrosis of the marrow; only fatty marrow was present.
4. In the same patient several joints exhibited arthritis deformans, raising the question of a possible bacteriological relationship between the two conditions.
Cultures of cyst lining and contents and of synovia of the left elbow joint in the above reported case were negative. The age of the cyst and the small amount of synovia cultured may possibly account for this.