To investigate the frequency of unrecognized Bordetella pertussis infections in adults, we performed IgA and IgG ELISA antibody studies with four B. pertussis antigens—i.e., lymphocytosis-promoting factor, filamentous hemagglutinin, pertactin, and fimbriae-2—in 51 health care workers from whom six consecutive yearly serum samples (from 1984 to 1989) were available. Overall, 90% of the subjects had a significant increase in antibody (IgA or IgG) to one or more antigens between 2 consecutive years during the 5-year study period; 55% of subjects had evidence of two infections, 17% had three infections, and 4% had four infections. Infections occurred in all study years, with the following rates: 1984–1985, 32%; 1985–1986, 24%; 1986–1987, 40%; 1987–1988, 29%; and 1988–1989, 43% (P = .12). Some antibody rises may have been due to responses to cross-reacting antigens (Bordetella parapertussis, nontypable Haemophilus influenzae), but overall these data suggest that B. pertussis infections in adults are common, endemic, and usually unrecognized.