Oxidative damage plays an important role in the pathophysiology of diabetes and diabetic complications. Feline hemoglobin is uniquely susceptible to oxidative denaturation; therefore, Heinz body formation is a highly sensitive indicator of in vivo oxidative stress in this species. Heinz bodies also contribute to anemia. We investigated hematological and clinical biochemical changes in 30 cats with spontaneous diabetes mellitus (as compared to 15 healthy control cats) and evaluated the relationship of these changes to erythrocyte oxidative damage. Cats were categorized as ketoacidotic or nonketoacidotic based on their clinical presentation and the presence of urine ketones. Ketoacidotic cats had significantly (P = .0009) more Heinz bodies (28.3% +/- 9.1%) than nonketotic diabetic cats (6.5% +/- 1.60%) and healthy control cats (0.6% +/- 0.2%). Percent Heinz bodies in diabetic cats directly correlated with plasma beta-hydroxy-butyrate concentration (r = .622; P = .0002), as well as with serum chloride concentration (r = -0.576; P = 0.0009) and the number of monocytes (r = .536; P = .0023). Percent Heinz bodies were negatively correlated with erythrocyte glutathione concentrations. Erythrocyte membrane lipid peroxidation was slightly but not significantly increased in diabetic cats. There were no significant associations between percent Heinz bodies and degree of anemia, hyperglycemia, or glycohemoglobin. These data indicate that ketones are associated with oxidative hemoglobin damage in cats, and suggest that ketone metabolism, ie by cytochrome P450 2E1, may be a potential source of in vivo oxygen radical generation in animals with ketosis.