The diagnosis of atopic disease is often difficult in small children because of differences in symptoms and lack of specific and reliable laboratory tests. We evaluated the significance of four commonly used laboratory tests--blood eosinophil count, total serum IgE, and eosinophil and mast cells in the nasal smear--as indicators of atopy in 178 children aged 3 years. The children were followed from birth and examined at the age of 3 years. Symptoms of immediate hypersensitivity including atopic dermatitis, food allergy and pollen or animal allergy were recorded. Severe or obvious atopy correlated with the highest levels of serum IgE. A total serum IgE level higher than 150 U/ml was found to be strongly suggestive of atopic disease. A blood eosinophil count higher than 600 X 10(9)/l as well as an increased number of eosinophil and mast cells in the nasal smear were associated with atopy. On the other hand, normal levels of these laboratory tests did not exclude atopic disease. In other words, all of the tests were high in specificity, but low in sensitivity. Consequently, when small children's atopic disease is being diagnosed, emphasis can be laid on the elevated levels of serum total IgE, blood eosinophil count and eosinophil and mast cells in the nasal smear, all of which separately, but especially together, give valuable information on atopy.