Glucose-6-phosphate dehydrogenase (G6PD) deficiency predisposes affected individuals highly susceptible to oxidative stress, which is one of the risk factors for diabetes. To evaluate the relationship between blood level of G6PD activity and diabetes in Taiwan, blood G6PD activity was analyzed among 237 patients with diabetes and 656 healthy subjects. A significant difference in the distribution of G6PD activities as grouped by an increment of 100 U/10(12) red blood cells (RBCs) was observed between diabetic patients and healthy subjects. The odds ratio for diabetes was 1.46 (95% confidence interval = 1.11-1.92) for every decrement of 100 U/10(12) RBC G6PD activities in these subjects. The data indicate that low G6PD activity is another risk factor for diabetes.