Seventy newly diagnosed diabetic patients were enrolled in the study, including 37patients aged over 60 (elderly-onset diabetes, EOD group) and 33 patients under 60 (adult-onset diabetes,AOD group) ; 30 elderly healthy subjects served as controls. Serum melatonin levels were determined with ELISA and the expression of inducible nitric oxide synthase (iNOS) mRNA in peripheral blood leucocytes (PBL) was detected by reverse transcription polymerase chain reaction (RT-PCR). The results showed that serum melatonin levels were significantly lower in EOD group than those in control group [ (6. 6 ± 1.3 )ng/L vs. (7.7 ± 1.4) ng/L, P ＜0. 01 ], the expression of iNOS mRNA was significantly higher than that in AOD group [ (36 ± 3 )% vs. (30 ± 4)%, P ＜ 0. 01 ]; and i NOS mRNA was not detected in control group.Logistic regression analysis indicated that decreased serum melatonin levels and up-regulated iNOS mRNA might be independent risk factors for diabetes mellitus in the elderly.