Four trials of external quality assessment in diagnostic hematology were performed in 2009 with average 946 participating laboratories in Korea. We performed quality assessment for white blood cell count, hemoglobin, hematocrit, red blood cell count, platelet count, blood cell morphology, prothrombin time and activated partial thromboplastin time. The response rate was more than 98.2%. The coefficients of variation in hemoglobin, hematocrit and RBC count were stable but variable in platelet count and WBC count according to measuring cell counters. Test results of blood cell morphology showed variation among various cell morphologies.