Three subsequent studies were concerned with clinical, pathophysiologic and immunologic aspects of recurrent croup. The first investigation demonstrated that recurrent croup is a distinct disease entity which shares many clinical features with asthma and should be distinguished from viral laryngotracheobronchitis. A high incidence of airway hyperreactivity was found in children with recurrent croup in the second study. This specific type of hyperreactivity suggested an involvement of both the lower and the upper respiratory tract. The third study demonstrated a tendency towards low serum IgA levels in children with recurrent croup; hence immunoregulatory defects may be conducive to the development of this disorder.