An assessment of human pulmonary effects from long-term, low-level exposure to methyl isocyanate (MIC) has been undertaken. Serial pulmonary function data, cigarette smoking histories, and other information were available for over 400 workers from a large chemical facility. In addition, industrial-hygiene measurements had been made and were used to classify jobs according to level of MIC exposure. In some instances, work records were incomplete and workers' predominant job and extent of inferred exposures were therefore based on the ratings of their supervisors and coworkers. The availability of these data allowed us to evaluate the frequency of pulmonary impairment in workers according to the assumed four levels of MIC exposure. No specific or consistent pulmonary impairment was evident. Long-term, low-level exposure to MIC at the levels existing at this particular facility could not be shown to be producing detectable effects on lung function.