The project examines possible causes for the high and rising frequency of allergies in many European countries. Infants from three different European regions are followed from birth to one year's age with assessment of environmental conditions (housing standards, family size, indoor milieu etc), feeding pattern, medical illness and treatment and establishment of major groups of aerobic bacteria in the normal intestinal micro flora. A study nurse keeps close contact with the families to minimize dropout and to assure proper handling of samples. A team of paediatric allergologists including specialists in paediatric gastroenterology examine all infants at 1.5 and 3 years' age for signs of allergicsensitization. Different allergic manifestations are related to known risk factors for allergy, other environmental conditions, and intestinal colonization pattern using multivariate analysis.