Certain disorders of the immune system seem to be associated with skeletal defects. The association was first recorded by McKusick et al. (Bulletin of the Johns Hopkins Hospital 116:285-326, 1964). A number of relationships between lymphocytes and osteocytes can be proposed. These include a common environment for development, common metabolic needs and effects upon osteocytes by products (cytokines) elaborated from lymphocytes or monocytes during immune responses. Thus, bony defects of varying degrees of severity are seen in short-limb dwarfs, cartilage-hair hypoplasia, and adenosine deaminase (ADA) deficiency. Cytokine activation of osteoclasts accounts for the lytic lesions seen in malignancies and the excessive bone resorption which accompanies autoimmune disorders such as rheumatoid arthritis. Correction of primary immune deficiency is accomplished by bone marrow transplantation. If the bony abnormality is subtle (as in some cases of ADA deficiency) the skeletal problem is resolved; if the bone defect is major as in short-limb dwarfism, no improvement is seen.