We studied isolates of nontypable Haemophilus influenzae from cultures of nasopharynx and middle ear fluid (MEF) done simultaneously on children with otitis media. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of outer membrane protein (OMP) patterns demonstrated that in 16 of 19 pairs, the nasopharyngeal and MEF strains were identical. With four monoclonal antibodies to lipooligosaccharide (LOS) determinants, 17 of the 19 pairs were identical. Thus the pathogenesis of otitis media due to nontypable H. influenzae appears to involve spread of the bacteria from the nasopharynx to the middle ear. Analysis of middle ear isolates from four children with recurrent otitis media caused by nontypable H. influenzae indicated that the recurrent episodes were caused by reinfection with different strains rather than by persistence of the same strain. OMP and LOS analysis of strains from two sisters with concurrent otitis media suggested that person-to-person transmission of nontypable H. influenzae can occur among children.