Haemophilus influenzae, one of the bacteria comprising the commensal flora of the human upper respiratory tract, is also pathogenic and causes both localized and invasive (septicemic) infections. The major focus of attention and research has been on infections caused by serotype b organisms, which cause several life-threatening illnesses in children, including meningitis and acute respiratory infection (ARI; e.g., epiglottitis, pneumonia). Type b polysaccharide-protein conjugate vaccines are at an advanced stage of development and implementation; however, these vaccines will not protect against noncapsulated (nontypable) strains of H. influenzae or strains expressing capsular polysaccharides other than serotype b, strains which cause a substantial proportion of ARI (especially pneumonia) among infants, particularly in developing countries. The magnitude of this problem, which contributes to many thousands-perhaps millions-of deaths each year, emphasizes the need for research on the epidemiology, pathogenesis, virulence factors, immune mechanisms, and forms of treatment relevant to ARI caused by H. influenzae in infants and implies that such studies should be given a high priority.