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The adherence to human epithelial cells, biotype and capsular type of 175 Haemophilus influenzae cultured from the upper respiratory tract were studied in a prospective study of children with recurrent otitis media. Forty-three children who had >2 episodes of acute otitis media (AOM) during the first year of life were followed for at least 1 year. Cultures of the oropharynx were done periodically, and the middle ear fluid (MEF) was cultured at the time of AOM.was recovered from MEF in 44% of the 136 AOM episodes recorded. Thirty-one children had at least one episode of AOM caused by H. influenzae; the remaining 12 children, designated as “controls,‘’ had no otitis or had AOM caused by other organisms. The possible differences between carriage and infection strains were evaluated by comparison of MEF and oropharyngeal isolates, by pairwise comparison of MEF and oropharyngeal isolates and by pairwise comparison of multiple isolates from each host recovered at the time of AOM and during infection-free intervals. No significant differences in patterns of adherence, capsular type or biotype were found. The lack of correlation between these characteristics and infection suggests either that H. influenzae organisms have determinants of virulence yet to be defined or that variations in host susceptibility permit infection by the strain colonizing the upper respiratory tract. Adherence per se may be less important in the development of infection than in establishing and maintaining colonization within the host.