Reactive dyes have gained extensive use in recent years. This paper reports four cases of immediate-type occupational allergy to these compounds. All the patients had had symptoms of asthma and allergic rhinitis, and had been weighing dyes for a minimum of 2 years. The skin prick tests were positive, and nasal and bronchial provocation challenges also produced positive immediate reactions. A high serum titre of specific IgE could be demonstrated for at least one dye by the radioallergosorbent test. The identification of specific IgE shows that the mechanism of the hypersensitivity is immunological, reactive dyes acting probably as haptens.