Whooping cough is an infectious disease caused by Bordetella pertussis. Particularly children under the age of six months can be severely affected by the infection. Severe leukocytosis may lead to thrombosis and pulmonary hypertension and eventually circulatory failure and death. This a case report of a three-week-old girl with malignant pertussis, who due to respiratory insufficiency was mechanically ventilated, and her severe leucocytosis was treated with exchange blood transfusion. Whooping cough may partially be prevented with efficient vaccination programmes.