The PIAMA birth cohort was set up with two main aims: to investigate the effect of mite-allergen avoidance on the incidence of childhood asthma, and to assess lifestyle and environmental risk factors for childhood asthma. The first study aim was evaluated through an intervention study, whilst the second study aim was investigated via a natural history study. Mothers were recruited into the study from 1996 to 1997, through 52 antenatal clinics in three different regions of the Netherlands. Over 4,100 pregnant women were recruited into the cohort, resulting in a sample of over 3,900 children at baseline. Data was first collected from mothers during pregnancy, with follow-up after birth at 3 months and yearly from age 1 to 8 years. Subsequent follow-ups at ages 11, 14, 17, 20 and 23 to 25 years have also been conducted.