In order to investigate the relationship between the pattern of response (immediate, late and dual) to specific bronchial challenge test with plicatic acid or red cedar extract and the clinical features of asthma, 332 patients with-asthma induced by western red cedar dust were examined at the time of diagnosis. Specific challenge test induced in thirty-one patients (9.3%) an isolated immediate reaction, in 144 patients (43.4%) an isolated late reaction and in 157 patients (47.3%) a dual reaction. Patients with a dual reaction had a longer period of exposure to red cedar dust between the onset of the respiratory symptoms and the time of the definitive diagnosis, a lower FEF 25–75% and a greater degree of non-specific bronchial hyperresponsiveness compared to patients with isolated immediate or isolated late reactions; the difference in bronchial hyperresponsiveness to methacholine among the three groups persisted when the values were adjusted for the different baseline value of FEV1. There was no difference in the prevalence of specific serum IgE antibodies to plicatic acid-human serum albumin conjugate among the three groups of patients with different type of response to red cedar. Except for the greater degree of non-specific bronchial hyperresponsiveness, patients with isolated late reactions were not different from those with isolated immediate reactions in other clinical findings. These findings indicate that a dual reaction in patients with occupational asthma due to simple chemical agents is indicative of a greater severity of disease at diagnosis. The pathogenetic mechanism of various types of asthmatic reaction is unknown and it is likely to be different between isolated immediate and isolated late reactions.