The aim of this study was to evaluate the ferric-reducing ability of serum (FRAS), paraoxonase 1 (PON1), ceruloplasmin serum oxidase activity and hsCRP level in patients with type1 diabetes mellitus without and with diabetic retinopathy. The study was performed in 76 patients with type 1 diabetes mellitus, 35 without diabetic retinopathy (group 1) and 41 with preproliferative and proliferative retinopathy (group 2). Control group consisted of 35 nondiabetic, age-, gender-, body mass-matched healthy volunteers who came to the outpatient clinic for a routine health check-up. We evaluated FRAS using the method described by Benzie and Strain; PON1 by kinetic spectrophotometric assay with paraoxon as substrate and ceruloplasmin using its oxidative activity with 3-phenylenodiamine as substrate. CRP was measured with a high sensitive enzyme immunoassay. PON1 activity was significantly decreased in patients with diabetic retinopathy (227.66 +/- 123.57 U/l) when compared with control (312.04 +/- 129.77 U/l). FRAS was significantly decreased in group 2 (439.33 +/- 79.87 micromol/l) when compared with group 1 (522.79 +/- 167.56 micromol/l) and control (529.80 +/- 81.99 micromol/l). Ceruloplasmin activity was significantly elevated in group 1 (58.36 +/- 22.56 U/g protein) when compared with control (45.22 +/- 14.96 U/g protein). We have found significant increase in hsCRP level in group 2 (3.71 +/- 2.47 mg/l) when compared with group 1 (1.75 +/- 1.01 mg/l) and control (0.57 +/- 0.46 mg/l). The PON1/CRP ratio in control group was significantly increased when compared with diabetic patients and was significantly decreased in group 2 compared with group 1. We have not found gender-dependent difference in studied parameters in both control and in study groups. We have found tendency to decrease the serum activity of FRAS and hsCRP in elder patients but the difference was significant only in group 2. FRAS and PON 1 activity is decreased in patients with type 1 diabetes mellitus with presence of diabetic retinopathy which confirms that oxidative stress could play a role in pathogenesis of diabetic retinopathy. Significantly elevated levels of hsCRP in diabetic patients with the presence of diabetic retinopathy compared with patients without diabetic retinopathy providing a link between inflammation and the development of microvascular complication of diabetes. Because of the significant difference in PON1/CRP ratio between patients without and with the presence of diabetic retinopathy, it seems that PON1:CRP ratio may be used as a biochemical marker for progression of retinopathy. The link between the antioxidant concentration, inflammation and the development of diabetes complications needs further longitudinal studies in order to confirm our findings.