Of the three etiologic types of infections croup the most frequently occurring type in this study was so-called "virus" croup. The pathology of so-called "virus" croup is distinct from that of croup due to H. influenzae type B and C. diphtheriae. It is similar to viral influenza in man and viral laryngotracheitis in chickens. Clinically "virus" croup is a progressive disease with downward spread of the infection in the respiratory tract. The inflammation can extend to various regions resulting in the clinical forms of the disease known as laryngitis, laryngotracheitis, or laryngotracheopneumonitis. The mortality increases as the disease spreads down the respiratory tract. There is no well-founded specific treatment for "virus" croup. However, sulfadiazine should be given to all infectious croup patients until cultures of the rhinopharynx and blood assure the absence of H. influenzae type B. Continued chemotherapy should be given all patients severely ill with the disease. The most frequent and severe complication of "virus" croup is some form of extraalveolar thoracic air. The most common cause of death is respiratory embarrassment.