The Prevention and Incidence of Asthma and Mite Allergy (PIAMA) birth cohort study was initiated in 1996. Children born to allergic mothers were enrolled in a double-blind placebo-controlled trial for evaluating the use of mite-impermeable mattress and pillow covers. Children born to allergic and non-allergic mothers were enrolled in a 'natural history' study to assess the role of environmental and dietary risk factors for the development of allergic disease in childhood. Recruitment started by distributing a validated screening questionnaire among >10,000 pregnant women during their first visit to a prenatal health clinic. Allergic mothers-to-be were invited to participate in the intervention study. Allergic, and a random sample of non-allergic, mothers-to-be were invited to participate in the 'natural history' arm of the study. In the intervention study, homes were visited before birth, 3 months after birth, and 12 months after birth for the collection of dust samples from floors and mattresses. In addition, the homes of about one-third of the children in the 'natural history' part of the study were visited for dust collection when the children were 3 months of age. The intervention study started with 855 participants and the 'natural history' study with 3,291 participants. Follow-up at 3 years of age has now been completed with satisfactory compliance (>90%). A medical investigation and home visit at 4years of age are nearing completion. Preliminary results show that mite-allergen levels were lower than found in previous Dutch studies, and that the intervention measure had a significant effect on mite-allergen levels, without important clinical benefits up to age 2 years old. The allergic families lived in homes with fewer 'triggers' such as pets, smoking and carpets than the non-allergic families, regardless of the intervention. The ongoing PIAMA cohort study will probably reveal useful information concerning effects of allergen load and reduction in the setting of a relatively low mite-allergen exposure, as well as other variables on the development of allergic manifestions and asthma.