The aim of the KOALA Birth Cohort Study in the Netherlands is to identify factors that influence the clinical expression of atopic disease with a main focus on lifestyle (e.g., anthroposophy, vaccinations, antibiotics, dietary habits, breastfeeding and breast milk composition, intestinal microflora composition, infections during the first year of life, and gene-environment interaction). The recruitment of pregnant women started in October 2000. First, participants with 'conventional lifestyles' (n = 2343) were retrieved from an ongoing prospective cohort study (n = 7020) on pregnancy-related pelvic girdle pain. In addition, pregnant women (n = 491) with 'alternative lifestyles' with regard to child rearing practices, dietary habits (organic, vegetarian), vaccination schemes and/or use of antibiotics, were recruited through organic food shops, anthroposophic doctors and midwives, Steiner schools, and dedicated magazines. All participants were enrolled between 14 and 18 wk of gestation and completed an intake questionnaire on family history of atopy and infant care intentions. Documentation of other relevant variables started in the pregnant mother and covered the first and third trimester as well as early childhood by repeated questionnaires at 14-18, 30, and 34 wk of gestation and 3, 7, 12, and 24 months post-partum. A subgroup of participants, including both conventional and alternative lifestyles, was asked to consent to maternal blood sampling, breast milk and a faecal sample of the infant at 1 month post-partum, capillary blood at age 1 yr, venous blood and observation of manifestation of atopic dermatitis during home visits at the age of 2 yr (using the UK working party criteria and the severity scoring of atopic dermatitis index), and buccal swabs for DNA isolation from child-parent trios. From the start, ethical approval and informed consent procedures included gene-environment interaction studies. Follow-up at 3 and 7 months post-partum was completed with high response rates (respectively 90% and 88% in the conventional group, and 97% and 97% in the alternative group). The home visits at 2 yr of age will be completed in 2005. Preliminary results show that we have succeeded in recruiting a large population with various lifestyle choices with a fairly large contrast with regard to dietary habits (including organic foods, vegetarian diet), vaccination schemes and/or use of antibiotics. We have also been able to collect a large number of faecal samples (n = 1176) and capillary blood samples at age 1 yr (n = 956). Furthermore, a large proportion of the participants have consented with genetic studies. Mid 2006 we expect to report our first results on the relationship between the various exposures in early life and childhood atopy. An outline of the focus and design of the KOALA Birth Cohort Study is presented.