It is our purpose at this time to report a fatal case of acute lymphatic leukemia, and another nonfatal case, in which a diagnosis of infectious mononucleosis was made. These cases are being reported for the purpose of comparison; hence a brief analysis of the clinical course, together with the morphology of the blood in both, should not be devoid of interest. The differential diagnosis should, ordinarily, not be difficult. However, confusion frequently occurs between acute leukemia and infectious mononucleosis. A correct diagnosis will usually be made if the patient is kept under observation for a period of time, and by repeated study of stained blood smears. The occurrence of acute leukemia with remissions renders a correct diagnosis extremely difficult. One not infrequently encounters atypical cases of idiopathic purpura, as well as agranulocytic angina, presenting blood pictures, clinical symptoms, and objective evidence that are strikingly similar, and leading one to