In order to diagnose and monitor patients with allergic diseases, we studied parameters which reflect not only the amount of eosinophils but also their state of activation. Morphologic features reveal additional information on the activity state of eosinophils. The number of nuclear lobes, cell size, amount of vacuoles and density of specific granules are considered characteristic features of eosinophils. We conclude that the size of eosinophils is a useful parameter to distinguish patients with and without eosinophilia. The treatment with corticosteroids did not affect morphological characteristics such as the number of vacuoles, granulation density, cell diameter and the nucleus/cell surface ratio, but the number of lobes per nucleus, a marker of eosinophil maturation, decreased significantly.