In the late 1990s, a birth cohort study was conducted in the cities of Delmenhorst, Wilhelmshaven and Leer, where more than 3,000 newborn children were recruited in five hospitals. The baseline survey in the clinics was followed by three follow-up surveys 6, 12 and 24 months later. The prime concern of the study was to estimate prevalences and to conduct analyses concerning the association between breastfeeding as well as exposure to pets and the occurrence of allergy symptoms. Children living together with a dog in the same household were at higher risk of disease only if a familial predisposition of allergic diseases was present - without such a familial predisposition a dog in the same household seems to reduce the risk for atopic diseases during the first 2 years of life. A protective effect due to long breastfeeding could be observed in our study in particular in case of a paternal history of allergic diseases, whereas an exclusive maternal history of allergic diseases seems to increase the risk. The concept of the study has proved itself. Contacting mothers in obstetrical departments in hospitals as well as in medical offices of self-employed pediatricians has proven to be very practicable. With comparatively little effort a relatively large cohort was recruited, which allowed us to analyze longitudinal data, adequately taking into account several confounders as well as effect-modifying factors.