Eighteen employees with lower respiratory symptoms later confirmed to be occupational asthma were referred for evaluation. All were employees of a single wood products plant using heated methylene diphenyl diisocyanate (MDI) in the manufacture of a synthetic wood product. Of the 18, 15 had no prior airway symptoms or other known bronchial injury, and 16 had positive methacholine bronchial provocation tests. All cases occurred during a 2.5-year period after exposure to a new manufacturing process using steam-heated MDI resin in a new manufacturing facility. Initially, employees developed symptoms related to the start-up process in the plant, with possible higher MDI exposures and probable higher resin temperatures. Later, most employees who developed new symptoms worked in areas of the plant where they were exposed mostly to heated boards. This suggests MDI sensitization arising at lower temperatures than previously considered likely for this substance.