708 unselected adolescents 15-17 years old were studied. They were physically examined, interviewed and skin prick tested with 12 common allergens. Questionnaires were distributed to the adolescents and their parents to investigate past and present symptoms and environmental influences. Emphasis was on the first year of life. Exposition to allergens (pollens and cat epithelium) during the first 6 months of life, eczema during the first year of life, and atopic heredity were found to be major risk factors in the development of positive skin tests and respiratory allergy apparent in the teenage period. Infection and wheezing during the first year of life were also risk factors, but less important.