Four recent outbreaks of pertussis in Massachusetts illustrate some features that contribute to the increased incidence of the disease. The outbreaks involved unimmunized groups of children with philosophical or religious exemptions from school or day-care immunization requirements and children and adults who were reluctant to undergo antibiotic prophylaxis or therapy. Parents and physicians should be aware that failure to immunize and to cooperate in follow-up preventive measures can have public health and potential medicolegal repercussions.