We performed a follow-up study on nine subjects with occupational asthma due to cobalt hypersensitivity confirmed by a specific bronchial provocation test. Both at diagnosis and at the two follow-up visits (1 and 3 years after the diagnosis, respectively), we graded the severity of asthmatic symptoms and the need for antiasthma treatment and performed lung function tests, measurement of airways responsiveness to methacholine (PD15) and skin tests. The evolution of the disease was established on the basis of the increase or decrease in asthmatic symptoms and the requirements for medication, FEV1 and PD15. After cessation of exposure, two subjects recovered, five improved and one remained stable; the subject who carried on working in the same environment deteriorated. The improvement in symptoms, medication need and lung function occurred earlier and in a larger number of patients than the improvement in methacholine hyperreactivity. At follow-up, the inhalation test with cobalt was repeated on three subjects who had stopped exposure and were asymptomatic and again they had bronchoconstriction. Our data show that early diagnosis and early removal from exposure after the onset of asthma are important factors for a favourable evolution of the disease and that specific and non-specific bronchial hyperresponsiveness can persist even in asymptomatic subjects no longer exposed to the sensitizing agent.