During May and June 1993, 10 patients and 5 members of the clinical staff at a hospital in California were diagnosed with Bordetella pertussis infection. In addition to erythromycin prophylaxis, 630 (48%) of 1330 staff members received a half dose of acellular pertussis vaccine with tetanus and diphtheria toxoids (DTaP). To identify side effects of the vaccine, a questionnaire was completed by 344 (54%) of 630 vaccinated staff. Side effects were reported by 117 respondents (34%); 64 were classified as mild (local reaction at injection site) and 50 as moderate (systemic complaints or local reaction resulting in limitation of arm movement). Three vaccinees (< 1%) reported missing 1 or more days of work because of their symptoms. Local reactions at the injection site occurred in 100 (29%), systemic symptoms in 38 (11%), and limitation of arm movement in 18 (5%). This study indicates that use of half dose of DTaP in adults appears safe and should be considered as an adjunct to chemoprophylaxis during institutional outbreaks.