A case of occupational asthma in a wool and cotton dyer handling reactive dyes is described. The patient was skin positive, but asymptomatic to house dust mites. A bromoacrilamidic dye (Lanasol Yellow 4G) has been identified through chamber inhalation challenge as being responsible for the sensitization. A very short (4-minute) exposure produced a severe immediate obstructive ventilatory defect followed by arterial hypotension and urticaria. Bronchial hyperresponsiveness as tested through metacholine challenge was absent both in basal conditions and after the dye challenge. Both prick and patch test for the dye were positive in the absence of any sign of contact dermatitis.