On September 16, 2002, the Crawford County Health Department (CCHD) reported to the Illinois Department of Public Health (IDPH) four cases of cough illness among workers at an oil refinery (total worker population: 750) in Crawford County, Illinois. On August 14, a worker aged 39 years reported to the plant's health unit with a cough lasting 14 days. On the same day, the worker's supervisor aged 50 years visited the health unit for a paroxysmal cough of 3 days' duration and an incident of cough syncope. Both patients were referred to private health-care providers; blood samples from both patients had serologic test results suggestive of recent Bondetella pertusis infection, and CCHD was contacted. On September 18, IDPH and CCHD initiated active surveillance and case investigations. This report summarizes the results of that investigation, which found that during August 1-October 9, pertussis was diagnosed in 15 (10%) of 150 oil refinery workers from two separate operations (n=95) and maintenance (n=55) complexes, who were linked by contact with the ill supervisor. Through enhanced case finding, 24 cases of pertussis, 21 (88%) of which occurred in adults aged > or = 20 years, were identified in this outbreak, underscoring the need to recognize this highly infectious disease in adults and to improve national diagnostic and preventive strategies.