The aims of this prospective and longitudinal project are to establish crucial risk factors for asthma and other allergic diseases in childhood, and to study factors of importance for prognosis at already established allergic disease. Socio-economic factors, such as inequality in health, are also to be addressed. The project started in February 1994. To reach sufficient power, 4,000 children had to be included. In November 1996, this number was reached (4,093). Inclusion in the study was made at 3–4 months of age. At that time, and before induction of allergic disease/asthma of the child, a questionnaire focused on exposure, genetics and socio-economic factors was answered. Settled dust was sampled for later analysis of furred animal and mite allergens. When the children were aged both 1 and 2 years, their parents were asked to fill in new questionnaires focusing on respiratory and allergic (skin, gastrointestinal) symptoms, but also key variables of exposure. Cases with asthma are identified and, for every case, two matched controls drawn. During the following winter, the homes of cases and controls were investigated and the temperature, indoor humidity, air change rate and NO2 measured. Two hundred cases (5%) were expected to be identified during the first 2 years of the children's lives. Some 479 homes have now been investigated and 97.7% of the original 4,093 children still remain in the cohort. The 2-year symptom follow-up ended in November 1998. The 4-year follow-up started on 1 September 1998 and was planned to be finished in June 2000. Questionnaires (allergic and respiratory symptoms, key variables of exposure at home and day care) are sent out to all 4,093 families. All children are invited for examination, lung function tests (PEF, flow-volume, MVV and oxygen clearance) and physical performance. Blood is taken from all children (20 ml). Allergy screening is performed and specific IgE examined. Blood cells will be frozen to allow for later DNA extraction. In subsets (children with any allergic and/or respiratory manifestation and controls), markers of inflammation in blood and urine will be examined, as well as eosinophils in nasal smear. Interviews are carried out to assess the severity of asthma, type/periodicity of health care given, asthma medication and parental sick leave when appropriate. As a separate project, financed by the EU, outdoor pollution as risk factors for asthma and allergies are to be studied within the BAMSE cohort. A follow-up of 8–9 years is underway.