To estimate the incidence of pertussis, a prospective study was done among members of a managed care organization in Minneapolis/St. Paul, Minnesota. Of 212 patients 10-49 years old enrolled from January 1995 through December 1996, 8 were found to be culture positive, 10 were found to be positive by polymerase chain reaction assay, 13 had a > or =2-fold increase in IgG or IgA to pertussis toxin (PT), and 18 had IgG to PT in a single serum specimen > or =3 SD above the mean of an age-matched control group. At least 1 positive laboratory test result for pertussis infection was found in 27 (13%) patients, among whom the duration of cough illness was a median of 42 days (range, 27-66 days). On the basis of any positive laboratory result, the estimated annual incidence of pertussis was 507 cases per 100,000 person-years (95% confidence interval, 307-706 cases). Bordetella pertussis infection may be a more common cause of cough illness among adolescents and adults than was recognized previously.