Few studies investigate how environmental factors in childhood may influence adult respiratory health. The European Community Respiratory Health Survey (ECRHS) is a longitudinal multi-centre study of Western world adults, including retrospective assessment of early life factors. Analyses of the ECRHS showed both beneficial and harmful long-term effects of childhood factors on adult asthma, allergy and lung function. Childhood pets were associated with less adult atopy and hay fever; beneficial effects were also indicated for growing up on a farm and for early exposure to other children. The findings have contributed to further development of the hygiene hypothesis and further understanding of the mechanisms relating microbial stimulation to allergy; however, the public health consequences may be limited. Harmful effects of early life factors on adult asthma and lung function were indicated for severe respiratory infections early in life, parental smoking and long-term dog keeping. Intervention with regard to parental smoking and vaccination against common lower airways infections may improve respiratory health in the population. Thus, early life environment had permanent beneficial and adverse effects on adult respiratory health. The multi-centre structure of the ECRHS, the large sample size, the extensive information about each participant and follow-up until the age of 56 years, have given the basis for convincing conclusions, and made possible publication of unsuspected findings in spite of the problems related to adult recall of childhood events. The ECRHS have contributed substantially to increased knowledge about the early life origins of allergy and obstructive pulmonary disease, providing a basis for prevention.