Longitudinal, community-based, multicenter study. Those subjects who took part in the first occupational asthma study survey were re-contacted in 1999, to determine risk factors for incidence and remission of asthma, other respiratory symptoms, atopy, changes in bronchial responsiveness and lung function. Subjects were asked to perform a forced spirometry, methacholine challenge and to provide blood samples for IgE testing. A questionnaire administrated by trained interviewers was delivered requesting information on respiratory symptoms and diseases, socio-demographic factors, tobacco smoking, complete occupational history since the last survey, environmental exposures, family history, diet, treatment and use of health services. Modular occupational questionnaires were developed for subjects employed as cleaners, homemakers, welders, metal workers, nurses, and subjects exposed to organic dusts in the paper and textile industries.