Oxidative stress was compared in plasma of 15 recently diagnosed (<2 mo) or 15 longstanding (>5 yr) type 1 diabetic patients with 15 healthy volunteers. Lipid peroxidation indices measured in plasma included thiobarbituric acid-reactive substances (TBARS), conjugated dienes, and lipid hydroperoxide (ROOH). The values obtained were corrected for phospholipid to minimize this as a confounding factor. In recently diagnosed diabetics, plasma conjugated lipid dienes were significantly elevated. However, in long-standing diabetics there was a marked increase in TEARS, conjugated dienes, and lipid hydroperoxide levels. Our findings showed increased oxidative stress in type 7 diabetics regardless of metabolic control and that conjugated diene measurement appeared to be the most sensitive bioindicator of oxidant stress in our population.