In view of some data of the literature suggesting that immunological changes could play a role in the pathogensis of diabetes mellitus and of its complications, we carried out a study of the lymphocyte populations of 44 diabetic subjects by means of E, EA, EAC rosettes and membrane immunofluorescence. A quarter of the patients had decreased E rosettes: all of these had long-lasting, insulin-dependent, poorly controlled diabetes. Some of these patients had also increased membrane IgG globulins. In 5 patients with recent onset, insulin-independent diabetes, E rosettes were increased. No significant changes of EA and EAC rosettes were observed. These findings suggest that changes of lymphocyte populations do not play any role in the pathogenesis of diabetes, while these may be the consequence of the metabolic disorder and these may favour the onset of complications of the disease.