The efficacy of two acellular pertussis vaccines was estimated for various clinical case definitions, with and without the requirement of culture confirmation, from a randomized trial in Sweden. Efficacy increased with duration of coughing spasms and when the case definition included whoops or whoops plus at least nine coughing spasms a day. After deletion of clinical cases not believed to be caused by pertussis, efficacies were closer to the higher values for culture-confirmed disease. Nonspecificity of the clinical criterion "21 days of coughing spasms with whoops" resulted in estimates of predictive value for pertussis of 85% for placebo recipients and 56% for vaccinees. We conclude that laboratory confirmation of suspected cases is needed in pertussis vaccine trials. A suggested case definition is 21 days or more of coughing spasms with confirmation by culture, serologic study, or household exposure to culture-confirmed pertussis.