Concentration of blood free-radicals in 19 normal subjects and 35 patients with NIDDM was estimated by electron spin resonance spectrometer. Serum lipid peroxide (Lpo) and vitamin E were measured with biochemical methods. As compared with controls, patients with NIDDM had significantly higher blood free-radical level and serum Lpo level. The results suggested that the metabolism of free-radicals was disturbed in NIDDM.