A 9-month-old nonimmunized white female patient presented with a paroxysmal cough and a white blood cell count of 114,000/mm3. A nasopharyngeal culture was positive for Bordetella pertussis. Hyperleukocytosis is a rare complication of pertussis and is attributed to lymphocytosis-promoting factor. Hyperleukocytosis with pulmonary leukostasis can result in significant hypoxemia, although it did not occur in the case presented.