Diabetic patients undergo a chronic oxidative stress. This phenomenon is demonstrated by low levels of reduced glutathione (GSH) levels. The NADPH used by glutathione reductase for the reduction of oxidized glutathione (GSSG) to GSH is also used by aldose reductase for the reduction of glucose to sorbitol through the polyol pathway. The competition for NADPH could be responsible for the decreased glutathione levels found in non-insulin-dependent diabetic patients. For this purpose, we investigated the effect of polyol pathway inhibition on the glutathione redox status in these patients. We measured GSH and GSSG levels in erythrocytes of non-insulin-dependent diabetic patients (n = 15) before and after 1 week of treatment with placebo, followed by 1 week of treatment with an aldose reductase inhibitor (tolrestat 200 mg/d). We found lower GSH levels (7.7 ± 1.4 μmol/g hemoglobin [Hb]), higher GSSG levels (0.35 ± 0.09 μmol/g Hb), and lower GSH/GSSG ratios (23.9 ± 7.7) in diabetics compared with controls (n = 15; 9.8 ± 0.8 μmol/g Hb, P < .001; 0.17 ± 0.02, P < .001; and 58.3 ± 9.1, P < .001, respectively). We did not demonstrate any statistical difference after 1 week of treatment with placebo. In contrast, the treatment with tolrestat induced a significant increase in GSH (8.9 ± 0.7 μmol/g Hb, P < .01), a decrease in GSSG (0.25 ± 0.06 μmol/g Hb, P < .02), and an increase in the GSH/GSSG ratio (37.3 ± 8.4, P < .01). These data strongly support the hypothesis that the polyol pathway plays an important role in the impairment of the glutathione redox status in diabetic patients.