The overall prevalence of occupational asthma is unknown. It has been estimated that 15% of all adult asthmatics suffer from asthma caused by occupational exposure. The prevalence of occupational asthma varies depending on the nature of the industrial agent, the concentration of exposure and working conditions. In Italy diisocyanates and flour dusts are the commonest causes (60%) of occupational asthma. The diagnosis of occupational asthma is made by confirming the diagnosis of bronchial asthma and by establishing a relationship between asthma and the work environment. History and measurement of non-specific bronchial hyperreactivity are useful to establish a clinical diagnosis, while skin (or serologic) tests with the appropriate extract may be useful in the identification of the responsible agent (high molecular weight compounds such as protein). With low molecular agents (e.g. diisocyanates, wood dust, colophony) the specific bronchial provocation test in hospital or the measurement of lung function tests at work ("stop-resume" test) are required. There are few follow-up studies of patients with occupational asthma; all of them show that only 40% (mean) of the patients completely recover after cessation of exposure. Atopy (one or more skin prick tests positive to common allergens) is not sufficiently discriminative for screening purposes when a person enters a new occupational environment where an occupational asthma agent is present. Preemployment screening is useful to define the higher risk workers: atopy may be used as an indication for more frequent or more strict monitoring in periodic medical surveillance.