The epidemiologic features and clinical spectrum of pertussis in the elderly are poorly understood. In October 1985, the Wisconsin Division of Health investigated an outbreak of pertussis in residents of a nursing home in rural Wisconsin. Clinical information and nasopharyngeal swab and acute- and convalescent-phase serum specimens were obtained from all consenting residents and employees. Of 105 residents, 38 (36.2%) were seropositive, including four who were culture-positive for Bordetella pertussis. Culture-positive residents (age range, 52-81 years) had cough lasting 43-54 days. Three of these residents had paroxysmal cough, and all four had cough that interrupted sleep; none of the residents had cough with apnea or vomiting, and all recovered without sequelae. Of six seropositive residents with clinical pertussis, five lived on the south wing of the facility. Of 104 employees, 8 (7.7%) were seropositive, but none were culture-positive for B. pertussis. The higher attack rate for residents and the clustering of clinical cases were consistent with ongoing transmission within the nursing home.