A foundry worker developed dyspnea and a restrictive breathing defect after being exposed to diphenylmethane diisocyanate (MD) at work. His symptoms and lung function impairment subsided a few weeks after he left work. Specific inhalation challenges with the offending agent induced general malaise together with a significant increase in body temperature and leukocyte count as well as a significant decrease in forced vital capacity and transfer factor. Total antibody activity to a MDI human serum albumin (MDI-HSA) conjugate was detected by ammonium sulfate precipitation. Specific IgG antibodies were demonstrated by the enzyme-linked immunoabsorbent (ELISA) technique. Thus, it was found that MDI exposure can cause hypersensitivity pneumonitis and induce the production of specific antibodies.