By the term leukemoid reaction is meant a blood picture that closely resembles, or is indistinguishable from, that of the various types of leukemia. At necropsy, however, leukemic changes in the organs and tissues of the body are lacking. Reactions of the leukemoid type have been recognized for many years, and it is now known that they occur with a wide variety of pathologic conditions. Our experience leads us to believe that they are much more frequently encountered than was formerly thought. Since it is essential that these reactions be distinguished from the leukemias, it is important that practitioners in all branches of medicine have a general idea concerning those diseases with which leukemoid reactions may be encountered. In attempting to group these leukemoid reactions, we realize that some of them might well be placed under two headings. As time goes on, however, other groups of cases will undoubtedly be