To examine the association between environmental exposure during the first two years of life and development of atopic disease later in child-hood, 115 children sensitized to airborne allergens and 54 non allergic controls were investigated. Fifty-seven of the atopic children were skin prick test (SPT) positive to house dust mites (HDM). Information on exposure was obtained from questionnaires. Parental allergy was a more important risk factor for development of atopy than any of the environmental risk factors. Exposure during the 2 first yr of life to environmental tobacco smoke (ETS) and home dampness was more frequently found among the HDM-sensitized children than among the controls (OR = 2. 8, CI 1. 0-7. 3 and OR = 4. 6, CI 1. 4-15. 0 respectively). A combination of these two factors seemed to be particularly harmful (OR = 8. 7, CI 2. 3-33. 1). Similar but less pronounced effects were seen for children with other allergies. Later in life exposure both to dampness and ETS was of the same magnitude in the atopic children as in the controls. It appears that exposure to environmental factors during the first few years of life may trigger for development of atopic disease and that interactions may be of particular importance.