AbstractPoly(ADP‐ribose)polymerase (PARP‐1), a nuclear enzyme activated by DNA strand breaks, is involved in DNA repair, aging, inflammation, and neoplastic transformation. In diabetes, reactive oxygen and nitrogen species occurring in response to hyperglycemia cause DNA damages and PARP‐1 activation. Because circulating mononuclear cells (MNCs) are involved in inflammation mechanisms, these cells were chosen as the experimental model to evaluate PARP‐1 levels and activity in patients with type 2 diabetes. MNCs were isolated from 25 diabetic patients (18 M, 7 F, age, 63.5 ± 10.2 years, disease duration 17.7 ± 8.2 years) and 11 age and sex matched healthy controls. PARP‐1 expression and activity were analyzed by semi‐quantitative PCR, Western and activity blot, and immunofluorescence microscopy. PARP‐1‐mRNA expression was increased in MNCs from all diabetic patients versus controls (P < 0.01), whereas PARP‐1 content and activity were significantly lower in diabetic patients (P < 0.0001). To verify whether low PARP‐1 levels and activity were due to a proteolytic effect of caspase‐3 like, the latter activation was measured by a fluorimetric assay. Caspase‐3 activity in MNCs was significantly higher in diabetic patients versus control subjects (P < 0.0001). The different PARP‐1 behavior in MNCs from patients with type 2 diabetes could therefore be responsible for the abnormal inflammation and infection responses in diabetes. © 2005 Wiley‐Liss, Inc.