The Michigan Surveillance System (MSS), a state based surveillance system for occupational asthma, was based on mandatory reporting of all known or suspected occupational diseases. Potential cases were reported to the system by physicians, hospitals, or the Michigan Department of Labor. The MSS was capable of providing sufficient data to enable estimates of the incidence of occupational asthma to be made, the characteristics of affected individuals to be described, and appropriate public health interventions such as workplace inspections to be made. During the first 6 years of operation of the MSS, from 1988 to 1994, 725 cases of occupational asthma were reported to the system. Most (76%) of the reports were received from physicians, 17.1% were from hospitals, 7.3% from workers' compensation records, and 3.5% came from other health professionals. There were 603 cases (83.2%) which had new onset asthma after a period of symptom free work exposure and 57 (7.3%) had aggravation of preexisting asthma. Sixty nine cases (9.5%) were diagnosed with reactive airways dysfunction syndrome. The reported cases represented an overall average incidence rate of occupational asthma in Michigan of 2.9 cases per 100,000 workers. When categorized by broad industrial sectors, the incidence rates in manufacturing, construction and mining, and service industries were 14.3, 4.3, and 0.8/100,000, respectively. Exposures to isocyanates and machining fluids were the two most common causes of occupational asthma, accounting for 33% of the cases. The authors conclude that despite its limitations, lack of objective tests to confirm diagnoses of occupational asthma and case underreporting, the MSS has proven successful in identifying new cases of asthma and identifying workers at risk for asthma.