Asthma has been increasing over the last two decades in the United States. The onset of asthma has also been increasingly reported as a result of occupational exposures to over 350 different agents. Work-related asthma (WRA) has become the most frequently diagnosed occupational respiratory illness. Epidemiologic studies from the United States reported WRA incidence rates of 29-710 cases per million workers per year and suggest that 10-25% of adult asthma is work related. Much can be learned about asthma in the general population from investigations of asthma in the workplace. Surveillance of WRA continues to highlight an important role for low molecular weight chemical sensitizers, as well as high molecular weight antigens. Additionally, recent reports implicate mixed exposures, including commercial cleaning solutions, solvents, and other respiratory irritants, as well as contamination in nonindustrial environments, including schools and offices. Investigations of WRA have demonstrated a clear dose-related increase in sensitization and symptoms for exposures to both chemical and protein sensitizers. High proportions of exposed working groups can be affected. Skin exposures may affect the likelihood of individuals developing respiratory symptoms. Atopy increases the risk of sensitization and illness from workplace exposure to antigens but not to chemical sensitizers. Irritant exposures can act as adjuvants among individuals exposed to sensitizing substances, increasing the proportion who become sensitized. Atopy might also be a result of irritant exposures in some persons. Occupational asthma often has important long-term adverse health and economic consequences but can resolve completely with timely control of exposures. Detailed study of such asthma "cures" may prove useful in understanding factors that influence asthmatic airway inflammation in the general population.