Epidemiological and laboratory studies have implied that the environment during early childhood is important for the risk of developing atopic disorders. In this study we analyzed the prevalence of asthma, hayfever and eczema among 1901 internationally adopted young men at the military-induction medical examination in relation to indicators of their early childhood environment. The adopted young men who came to Sweden before 2 years of age suffered from asthma, hayfever and eczema significantly more often than those who came to Sweden between 2 and 6 years of age; the risk ratios (RR) were 1.6, 2.5 and 2.1, respectively. The young men who were born in the Far East were identified as being particularly susceptible to the development of hayfever and eczema, with RRs of 1.3 and 1.7. This study demonstrates that the environment during the first 6 years of life has a profound influence on the risk of suffering from atopic disorders as young adults.