The limitations and usefulness of glycosylated hemoglobin (HbA1) as a means of assessing glycemic control in peritoneal dialysis (PD) and the importance of good glycemic control in diabetics on PD were analyzed. HbA1 levels may be elevated in renal failure because of either interference with the assay of HbA1 by carbamylated hemoglobin, which is formed by dissociation of urea to cyanate and condensation of cyanate with hemoglobin, or increased rate of formation of HbA1 secondary to acidosis or other uremic effects. Interference with the assay of HbA1 is limited to methods separating HbA1 from hemoglobin A by electrical charges and is not encountered when chemical or immunologic assays are used. The proposed effect of uremia on the rate of formation of HbA1 was refuted by in vitro studies and in vivo multifactorial analysis, which showed that blood glucose level is the only important determinant of HbA1 in patients with renal failure. HbA1 levels may be low in uremia because of shortened red cell survival and/or frequent transfusions of red cells, conditions rarely encountered in PD patients. In the great majority of the reported studies, HbA1 regardless of the method of assay used, correlated with direct glycemic indices in patients with renal failure treated conservatively, by hemodialysis or by PD. Poor glycemic control, indicated by high HbA1 levels, is associated with adverse outcomes, including prolonged hospitalization and shortened survival, in diabetics on PD. HbA1 is a useful tool in assessing blood glucose control in PD. Good glycemic control is important for diabetics on PD.