In osteoarthritis and rheumatoid arthritis the hardness of the subchondral bone of the medial tibial plateau is lower than in normals. In order to further analyse this study of the morphologic characteristics in bone from the mentioned region was carried out in 22 normals, 14 osteoarthritis and 12 rheumatoid arthritis. Specimens from these groups were subjected to a radiologic assay, a light microscopic investigation and an evaluation of the occurrence of intraosseous lipids. The normals showed a remarkable integrity of the subchondral trabecular network with advancing age only with slight osteoporosis and occasional sclerosis. In osteoarthritis there were osteoporosis, osteolysis, sclerosis and osteophytes all in good correlation to the grade of osteoarthritis present. In rheumatoid arthritis there were areas of rarefaction, fractures of the trabeculae, sclerosis and invasion of granulation tissue. The radiologic appearance corresponded well with the morphologic observation. No abnormal presence of lipids was encountered. This investigation supports the concept that the hardness of the subchondral bone of the medial tibial plateau much depends on the morphologic structure of the bone.