The occurrence of DNA fragmentation in lymphocytes obtained from alloxan-induced diabetic rats and diabetic patients was investigated. A high proportion of apoptotic lymphocytes in diabetic states may explain the impaired immune function in poorly controlled diabetic patients. Rat mesenteric lymph node lymphocytes were analysed for DNA fragmentation by using flow cytometry and agarose gel, and for chromatin condensation by Hoescht 33342 staining under different situations. Immediately after being obtained, the proportion of lymphocytes with fragmented DNA was twofold higher in alloxan-induced diabetic rats than in cells from control rats. After 48 h in culture, the occurrence of DNA fragmentation was also higher (81%) in cells from diabetic rats. Hoescht staining and fragmented DNA visualized in agarose gel were also higher in lymphocytes from alloxan-induced diabetic rats than in control cells. To investigate if this phenomenon also occurs in humans, blood lymphocytes from 14 diabetic subjects were examined. Similar results to those of rat lymphocytes were found in cells from diabetic patients immediately after being obtained and after 48 h in culture. The high occurrence of apoptosis in lymphocytes was accompanied by a reduced number of blood-circulating lymphocytes in diabetic patients. The involvement of low insulinaemia for the occurrence of apoptosis in lymphocytes was also examined. Insulin treatment markedly reduced the proportion of lymphocytes with fragmented DNA in alloxan-induced diabetic rats.