The state had more than 4,000 cases in 2004. New York was next with half that number. Illinois had more than 1,500 cases which was a 500 percent increase over 2003. Many other states had just single or double-digit figures.

Wisconsin Immunization Program director Dan Hopfensperger says he suspects other states aren't as aggressive about diagnosing whooping cough.

He says the state is examining the way it identifies and investigates cases and is comparing notes with other states, and Marshfield Clinic infectious disease specialist Matt Hall attributes Wisconsin's number to new and more testing.