Summary1. In a series of 100 cases of pneumonia in infants and children, it was found that the mortality was inversely proportional to the leukocyte count, except that in children having over 50,000 cells per c.mm.2. In this series of pneumonia, regardless of type, there is a relation between the lack of response on the part of the body in the production of leukocytes and a high mortality.3. Evidence is presented which indicates that the total leukocyte count offers a valuable method of approximating the reaction of the host to the infection, and hence may be utilized as a prognostic aid.4. Suppurative complications and extension of the pneumonia may be anticipated even in the presence of the primary pneumonia by the sudden increase of the leukocyte count.