The NACManchester Asthma and Allergy Study is a prospective study of the development of asthma and allergies in childhood. The subjects (995 children at age 3 years) were recruited in utero by screening parents in the antenatal clinic using skin prick testing and a questionnaire regarding allergic diseases. Children were assigned to risk groups according to parental atopic status (high risk, both parents atopic; medium risk, one parent atopic; low risk, neither parent atopic). A subgroup of those at high risk (with no pets in the home) was randomized to stringent environmental control (allergen impermeable covers for the parental and infant bed, hot washing of bedding weekly, HEPA vacuum cleaner, hard floor for the nursery), and the remainder followed a normal regime. The children have been followed prospectively. The environmental influences are very clearly defined. Measurements of environmental exposures include levels of house dust mite; cat and dog allergens during pregnancy and early life; pet ownership and exposure; childcare arrangements; number of siblings; vaccination uptake; thorough dietary questionnaire; and endotoxin exposure. Further unique objective outcome in the cohort is the assessment of lung function in preschool children using specific airways resistance, which at age 3 years clearly reflects both genetic and environmental influences.