Isocyanates are widely used in industry for the manufacture of polyurethanes. Although they are recognized as an important cause of occupational asthma, it is unclear whether asthma persists after avoidance of exposure. We have followed up 50 cases, all of whom had avoided exposure for at least 4 years. At follow-up, 82% continued to have respiratory symptoms and approximately half of these required treatment at least once per week. These results indicate that a significant proportion of those with isocyanate-induced asthma are likely to have persisting symptoms for at least several years after exposure is avoided.