Reduced levels of glycosylated hemoglobins (GHb) have been found to be closely related to red cell survival. We therefore studied the relation of this parameter to the clinical applicability in patients with hemolytic disease (n = 20). During a 5-week period we repeatedly measured severity of anemia, i.e., hemoglobin (Hb), packed red cell volume (VPRC), and red blood cell count (RBC), as well as reticulocytosis and parameters of red cell destruction such as serum concentration of lactic dehydrogenase (LDH) and bilirubin together with GHb. There was a weak correlation between simultaneously measured GHb and RBC (r = 0.5, P = 0.02), but none was demonstrable between GHb and Hb, VPRC, reticulocyte counts, LDH, or bilirubin. A much closer correlation, however, was found between actual GHb levels and RBC determined 3-5 weeks previously (r = 0.72, P = 0.001), as well as Hb (r = 0.56, P = 0.015), VPRC (r = 0.57, P = 0.013), reticulocyte counts (r = -0.63, P = 0.006), LDH ( r = -0.53, P = 0.02), and serum bilirubin concentrations ( r = -0.55, P = 0.016). Ghb was also significantly decreased in patients with consistently low values of reticulocytes when red cell destruction was demonstrable. These results show that GHb is a measure of red cell destruction and restitution, and thus may be usefull for long-term monitoring of patients with hemolytic disease.