Nasopharyngeal cultures and titer rises in paired sera were evaluated in a placebo-controlled pertussis vaccine efficacy trial. IgG ELISA for filamentous hemagglutinin (FHA) identified 30 (88%) of 34 placebo recipients and 33 (89%) of 37 vaccine recipients with culture-verified Bordetella pertussis infections, whereas IgG ELISA for pertussis toxin (PT) showed higher diagnostic sensitivity in the placebo group than in the vaccine groups. The CHO cell assay did not improve sensitivity. Children with Bordetella parapertussis infections had rises of titers of antibody to FHA of the same magnitude as children with B. pertussis infections. Sensitive serologic criteria, based on the intraassay variations, identified 105 additional culture-negative cases with significant titer rises in paired sera. IgG ELISA for FHA and PT and IgA ELISA for FHA were reliable assays, and bacterial isolation rates were lower in vaccine recipients than in placebo recipients with serologically defined pertussis.