We studied the influence of erythropoietin (EPO) treatment on hemoglobin A1c (HbA1c) levels under conditions which eliminate the effect of changes in the blood glucose concentration. HbA1c levels, blood glucose, hematocrit (Hct) and reticulocyte counts were serially measured every two weeks after starting or stopping EPO administration in 15 non-diabetic hemodialysis patients. EPO treatment significantly influenced HbA1c levels, and the more erythropoiesis fluctuated by changing the dose of EPO, the more HbA1c levels changed, though there were no significant changes in blood glucose levels during the study period. The changes in HbA1c during the 2-week period correlated inversely with both the changes in Hct during the same 2 weeks and the reticulocyte counts at that time. We concluded that the change in Hct should be kept in mind when the HbA1c level is evaluated in EPO-treated patients and a formula should be proposed to correct HbA1c levels based on the change in Hct or the reticulocyte count.