Six workers with occupational asthma due to toluene diisocyanate (TDI) were studied. For each worker a detailed clinical and occupational history was taken, and lung function measurement and skin intradermal tests for common allergens were carried out. Methacholine inhalation challenge was performed before TDI inhalation, and 8 h after TDI inhalation. Methacholine challenge was within normal limits when performed before TDI inhalation, but went into the asthmatic range after TDI inhalation. These cases provide evidence that asthma can be induced by toluene diisocyanate in the absence of airway hyperresponsiveness. They further demonstrate that an isolated negative methacholine inhalation test cannot be used to exclude sensitization to TDI. Screening and follow-up studies on workers exposed on TDI require serial measurements of airway responsiveness and of variable air-flow obstruction.