Two separate outbreaks of pertussis occurred within a three-month period on the pediatric units at the University of Colorado Medical Center. Secondary cases developed among adult members of the hospital staff, as well as among pediatric patients. While most adults were only mildly ill, two adults were seriously incapacitated. Pertussis agglutination titers were performed on sera from 341 adults. Results emphasize the susceptibility of adults to pertussis, and suggest that exposure to pertussis is high among pediatric house officers, medical students, and pediatric nurses as compared to the general population. In individual cases, difficulty may be experienced making a firm diagnosis of pertussis with available techniques. This contributed to the late identification of the outbreak, and emphasizes the need for improved reporting systems in hospitals.