Froelich, in 1897, was the first to report on the leukocytosis and lymphocytosis of pertussis. Students of the blood picture in whooping cough and authors of pediatric textbooks and laboratory manuals stress the diagnostic importance of these changes in the blood. Pertussis is the only disease of the respiratory tract in which they occur. Clinicians are often perplexed, however, by the blood picture early in this disease. Leukocytosis and lymphocytosis may be absent during the fortnight in which the signs and symptoms of the disease gradually become manifest. On the other hand, blood counts may appear elevated, with a preponderance of lymphocytes in young, coughing children who fail to develop the characteristic clinical picture. Many physicians have therefore abandoned blood counts as an aid in early diagnosis. In our work with pertussis, we, too, found the blood picture early in the disease very misleading. We have learned that it is