This paper describes a case of dermatomyositis occurring in a patient with agammaglobulinemia. A major manifestation of the disease process was subarachnoid hemorrhage, presumably due to central nervous system vasculitis. Typical changes of dermatomyositis were found in the skin, muscle, gastrointestinal tract, endocardium, epididymis, and brain. No immunoglobulins or β1 C-globulin were localized in the sections of skin or muscle. The failure of the patient's peripheral blood lymphocytes to respond to specific antigens in vitro suggests a qualitative disorder of cell-mediated immunity which may have predisposed to the development of dermatomyositis.