A series of examinations were conducted on the patients with leukemoid reaction who were abmitted to our Clinic for the period of from April, 1956 to June, 1961. These patients were suffering from the diseases other than malignant tumors. The following results were obtained. In the 9 cases showing the leukemoid reaction, the incidence of the reaction was about the same as that observable in patients with malignant tumors. Basic disease in these patients were agranulocytosis in 2 cases, 2 infectious diseases, 2 parasitoses and 3 myelofibroses. The blood pictures attributes to the leukemoid reaction varied, namely, consisting of either neutrophils, eosinophils or lymphocytes. This differed in case by case according to individuals. Only from several data such as past and present histories, clinical symptoms, blood pictures, bone marrow pictures etc., it may be said that sometimes it is rather difficult to diagnose a disease as a leukemia, while it is possible to distinguish it fairly accurately as a leukemia using the tissue culture technic. In the cases of myelofibrosis whose bone marrow puncture turned out to be a dry tap, the diagnosis was made by the tissue culture on a needle biopsy specimen of the spleen.