Interaction between diabetes mellitus and cellular immunity is reviewed in several aspects. Importance of cellular immunity in the development of type 1 diabetes mellitus is discussed. Experimental mice models of this disease are presented which after the evaluation of similarities of immunologic parameters provide clues to potential mechanisms of the pathogenesis of type 1 diabetes mellitus. In an animal model of spontaneous type 1 diabetes mellitus, the non-obese diabetic mouse develops a spontaneous T cell-mediated insulitis, which is followed by overt diabetes. Disease can be transferred to young non-obese diabetic mice with diabetogenic T cells from older, affected animals. T cells can recognize various islet antigens, including insulin and islet cell enzyme called glutamic acid decarboxylase. Induction of T cell tolerance to these antigens retards the onset of diabetes in mice. Transgenic mouse models of type 1 diabetes mellitus confirm above-mentioned facts. Later alterations of cellular immunity are under the investigation. Disorders of mobilization and chemotaxis, phagocytosis and adherence of polymorphonuclear leukocytes, monocyte function, defects of acquired cellular immunity are thoroughly discussed. Diabetes mellitus, particularly poorly controlled diabetes, impairs different functions and links of cellular immunity. Infections, which are found relatively more frequently in diabetic patients and against which primarily cellular immunity protects, are also reviewed.