Diagnosis of whooping cough by an enzyme-linked immunosorbent assay (ELISA) that measures serum IgG, IgM, and IgA antibody to the fimbrial hemagglutinin of Bordetella pertussis was compared to isolation from nasopharyngeal swabs in a prospective study. Of 77 patients with upper respiratory tract infections of unknown etiology in which B. pertussis infection could not be excluded on clinical grounds, 26 were culture-positive, including one for Bordetella parapertussis. All 26 patients were positive by ELISA except one asymptomatic erythromycin-treated patient (ELISA sensitivity, 96%). Among culture-negative patients, 24 additional patients were positive by ELISA. Thus, only about one half of the patients with whooping cough were identified by culture under optimal conditions. Positive titers of IgM antibody and/or high titers of IgG antibody in the first serum sample, allowing for a rapid diagnosis, were found only in 26 (53%) of 49 serologically positive patients. By combining culture testing with serology, rapid diagnosis was obtained in 41 (82%) of 50 patients.