The term of "leukemoid reaction" was introduced by krumbharr (1926) and he mentioned that a condition could not be differentiated by only peripheral blood picture. The authors reviewed 109,307 cases of peripheral blood examination of which 160 cases were selected. The selected 160 cases showed over 50, 000/cu mm in total white cell count or more than 2% of immature cells in differential count, respectively. The results were as followings: 1. The leukemia was in 84 cases (52%) and the leukemoid reaction was in 76 cases (48%), respectively. 2. The causes of leukemoid reaction in these cases were as followings ; infection in 32 cases (42%), metabolic disturbance with or without tissue necrosis in 11 cases (14.4%), intoxication in 11 cases (14.4%), malignancy in 8 cases (10.5%) severe hemorrhange in 6 cases (7.9%), and miscellaneous in 6 cases (7.9%). 3. In comparision of peripheal blood findings of leukemoid reaction with leukemia, anemia was prominent in leukemia than leukemoid reaction; in leukemia, mean hemoglobin value was 7.6 gm/dl. and hematocrit was 23.2%, and in leukemoid reaction, mean hemoglobin value was 10.7 gm/dl. and hematocrit was 33.6%, and also total white blood cell count were extremely higher in leukemia with average 82,600/cu mm than leukemoid reaction with average 22,100/cu mm.