Six polyurethane foam workers were shown to have TDI-induced asthma by specific inhalation challenge testing. All remained in the workplace in jobs with minimal TDI exposures. Mean and time-weighted average exposure concentration for the asthmatic group was 0.64 +/- 0.46 ppb, less than 5 percent of the permissible exposure limit and significantly less (p less than 0.01) than the mean TWA exposures occurring in the foam line and the finishing workers. Serial evaluation of the respiratory health of these six showed all persisted with respiratory symptoms, none had improvement in bronchial hyperresponsiveness to methacholine, and three had a greater than 15 percent decline in FEV1 on one of the days of spirometric testing over the five-year period from 1982 to 1986. Although we could not have predicted the outcome of asthma had these workers left the workplace and ceased isocyanate exposure entirely, occupational asthma persisted despite negligible ongoing TDI exposures.