Over a period of 6 years, 114 strains of Haemophilus influenzae and Haemophilus parainfluenzae were isolated from genital, mother-infant, or neonatal infections. Their serotypes, biotypes, antibiotic resistance phenotypes, and outer membrane protein (OMP) electrophoretic patterns were characterized and correlated with the various clinical outcomes. Genital H. influenzae and H. parainfluenzae appeared to behave mostly as opportunistic pathogens; for instance, 62% of the cases of endometritis or pelvic inflammatory disease were related to the presence of an intrauterine device. However, as seen clearly in one case, the strains may be sexually transmitted. The analysis of OMP patterns proved to be a very convenient method to seek evidence for the sexual origin of the infection. H. influenzae was more often involved in complicated genital infections than was H. parainfluenzae. Nontypeable and biotype II H. influenzae strains were the more frequent isolates, except in pelvic inflammatory diseases, in which biotype I prevailed, and in mother-infant infections, in which one-fourth of the cases were due to biotype IV. Characterization of H. influenzae isolates did not support a general concept of specific genital strains. However, strains of biotype IV clearly stood out with two characteristics: (i) a peritrichous fimbriation and (ii) a very peculiar homogeneous OMP pattern comprising an OMP of molecular weight approximately 18,000 unique to this biotype. These characteristics were also found in H. influenzae biotype IV strains isolated from genital infections in the United States and used as controls. H. influenzae biotype IV strains may thus correspond to a group somewhat adapted to the genital tract.