Disease diagnosis, age, sex, and selected hematologic variables were evaluated retrospectively in a population of feline patients with high number of circulating Heinz bodies. By comparing these cats with a control population and results of additional hematologic investigation on a subsample of the cats, we tested the hypotheses that endogenous Heinz body formation is increased in specific disease states and that endogenous Heinz bodies may contribute to anemia. There was strong correlation between diabetes mellitus, hyperthyroidism, and lymphoma and Heinz body formation. Diabetic cats, in particular, consistently had marked Heinz body formation. These diseases together accounted for nearly 40% of cats with Heinz body formation, but for less than 12% of cats of the control group. The PCV of cats with Heinz bodies (29.77 +/- 9.32%) was significantly (P less than 0.001) lower than that of control cats (35.33 +/- 8.08%). Polychromasia and punctate reticulocyte number were slightly increased in cats with Heinz body formation and correlated significantly (P less than 0.001) with PCV. A subsample of 13 of the cats had significant (P less than 0.006) inverse correlation between Heinz body percentage and erythrocyte reduced glutathione (GSH) concentration. Mean GSH concentration was significantly lower in cats with Heinz bodies, compared with that in a random cat population (5.28 +/- 1.67 mumol/g of hemoglobin vs 7.06 +/- 2.10 mumol/g of hemoglobin), in which GSH values followed normal distribution. Cats with Heinz body formation were older, and were more likely to be spayed.