Allergic diseases are common among small children, but it is still unclear how immunoglobulin E (IgE) antibodies to ambient allergens are distributed in a population-based prospective material of children at 4 years of age. The study is based on 75% (n = 4089) of all eligible children from northern Stockholm, born between 1994 and 1996 in pre-defined geographical areas. Data on exposure and outcome were obtained by parental questionnaires when the child was 3 months and 4 years of age. Of the 92% who responded to the 4 years of age questionnaire, serum was obtained in 88% of these children for analysis of IgE antibodies performed with Pharmacia CAP system (Phadiatop and food mix fx5). An antibody level > or =0.35 kUA/l was considered as positive. A positive Phadiatop or fx5 was found in 24% of the 4 years old children. A rather poor correlation was found between the two tests (r = 0.39). Occurrence of IgE antibodies > or =3.5 kU/l for both Phadiatop and fx5 in combination could predict any suspected allergic disease [asthma, rhinitis, atopic eczema dermatitis syndrome (AEDS) and allergic reaction to food] to 97.4%. However, the presence of > or =3.5 kUA/l of Phadiatop or fx5 used as single tests only, was far less efficient to predict any allergic disease. The two mixes of airborne and food allergens were also associated, not only to the severity of the allergic disease in terms of number of organ involved, but also to the severity of recurrent wheeze, in particular in boys with a positive Phadiatop who exhibited significantly limited peak flows compared to those with a negative test. Already at the age of 4, one child in four is sensitized to an allergen as assessed by Phadiatop or food mix (fx5). The presence of IgE antibodies seems not only to predict allergic diseases in this age group, but also relates to severity of such diseases, in particular to asthma. Notable, there was a poor correlation between Phadiatop and fx5 that needs to be considered when identifying allergic diseases in young children. The study demonstrates that quantification of IgE antibodies in blood may be beneficial, not only to diagnose allergic diseases in young children, but especially to serve as a marker of severity of asthma.