An outbreak of pertussis was recognized and investigated in a home for neurologically impaired people. Of 66 residents, 44 (67%) had evidence of recent pertussis infection, although only 12 (27%) had respiratory symptoms. Pertussis was diagnosed by culture, direct fluorescent antibody testing of nasal secretions, agglutinating antibody titer, pertussis antitoxin titer, IgG antibody to pertussis toxin, IgA antibody to pertussis toxin, IgA antibody to filamentous hemagglutinin, or IgG antibody to filamentous hemagglutinin. No single test identified more than 66% of people with pertussis. Of those with positive serologic findings, 95% had a positive test result for IgA antibody to filamentous hemagglutinin or for IgG antibody to pertussis toxin or for both. Pertussis occurred in both immunized and unimmunized residents. Seven carriers were identified; these residents had positive cultures or positive direct fluorescent antigen test results but negative serologic findings. Treatment of residents and caretakers with erythromycin halted the outbreak.