One hypothesis for the reduction in oxidative killing of neutrophils in diabetic patients is that increased polyol pathway activity during hyperglycemia reduces intracellular levels of nicotinamide adenine dinucleotide phosphate (NADPH), resulting in the reduction of neutrophil superoxide production during the respiratory burst. To test this hypothesis, we assessed the effect of tolrestat, an aldose reductase inhibitor, on neutrophil respiratory burst activity (NRBA) in diabetic patients. We measured fasting plasma glucose (FPG), hemoglobin A1 (HbA1), and NRBA levels in 79 diabetic patients and 48 normal controls. NRBA was reassessed in 34 patients after 4 weeks of tolrestat or placebo treatment, in seven controls after 4 weeks of tolrestat treatment, and in seven patients after 4 weeks of blood glucose control. NRBA was determined by flow cytometry, which detected fluorescent 2',7'-dichlorofluorescein (DCF) in neutrophils formed from 2',7'-dichlorofluorescein diacetate (DCF-DA) during phorbol myristate acetate (PMA)-induced respiratory bursts. Diabetic patients showed lower NRBA than the normal controls (mean cellular fluorescence, 438 +/- 103 v 668 +/- 101, mean +/- SD, P < .001). NRBA in diabetic patients showed a negative correlation with HbA1 (r = -.336, P < .005). Tolrestat treatment for 4 weeks in 17 patients restored the reduced NRBA to an almost normal level (relative NRBA, 0.55 +/- 0.20 v 0.99 +/- 0.36, P < .05) despite the fact that FPG level did not change (11.8 +/- 2.8 v 11.4 +/- 2.8 mmol/L). NRBA of these patients after tolrestat treatment was not significantly different from that of seven control subjects treated with tolrestat for 4 weeks. In 17 placebo-treated patients, there were no significant changes in NRBA and FPG level. The vigorous blood glucose control for 4 weeks in seven patients (16.6 +/- 2.1 v 8.6 +/- 2.3 mmol/L) also restored the reduced NRBA to almost normal (relative NRBA, 0.55 +/- 0.21 v 0.90 +/- 0.30, P < .05). The result that the reduced NRBA in diabetic patients was restored to almost normal either by tolrestat treatment or by blood glucose control strongly supports the hypothesis of this study.