Isocyanates, particularly Toluene Diisocyanate (TDI), have been the most common cause of occupational asthma for decades. Fifty workers with a diagnosis of isocyanate-induced asthma were followed up for a mean period of 8.4 years. Pulmonary function testing for FVC and FEV1, allergological tests, nonspecific bronchial challenge to methacholine and specific bronchial challenge to isocyanates were performed at the time of diagnosis and during follow-up. Data on symptoms and drug response were rated on a scale. Patients were subdivided into two groups based on persistence of (A; 13 patients) or removal from (B; 37 patients) isocyanate exposure. There were no significant differences in any characteristic between the two groups at the time of diagnosis. At follow-up, group B subjects showed significant improvement in symptoms, consequent reduction in use of medications and increase in PD20. None worsened, sixteen (43.3%) remained stable, twelve (32.4%) improved and nine (24.3%) became asymptomatic. The latter subjects were generally younger, they had shorter duration of symptoms and exposure to the agents, and longer removal from exposure, although none of these differences was significant. In 67% of cases they had an immediate reaction to TDI challenge. By contrast, the condition of group A patients deteriorated significantly during the follow-up period in terms of symptoms, pulmonary function parameters, PD20 and use of medications. Overall, seven subjects (53.8%) worsened during follow-up and none improved. Group B subjects were further divided into subgroups B1 and B2 based on duration of removal from exposure: B1 < 10 years, B2 > or = 10 years. There were no significant differences between the two subgroups, even though PD20 was higher in B2. The present study confirms that early diagnosis and immediate removal from exposure are crucial, though not always sufficient, for a favourable evolution of the disease.