Morphologic, instrumental (flow cytometric), cytochemical, ultrastructural, and chromosomal studies were performed in 21 cases of eosinophilic peroxidase deficiency that were observed in an area of northeastern Italy in the last 5 years. It was found that eosinophilic peroxidase deficiency occurred with a frequency of 1 case in 14,000 complete blood counts yearly, and thus is less rare than previously thought. Eosinophils appeared morphologically normal when examined using the light microscope, but ultrastructural study disclosed several aspecific granule alterations. In the first family studied, members with partial and total deficit were identified; in all the other cases, the enzyme deficit was total (negative cytochemical reactions and absence of dimethylaminoazobenzene-positive specific granules at the electron microscope), isolated (a single affected member in each family examined), and stable (persistent at long-term follow-up). Eosinophilic peroxidase deficiency was not correlatable with any particular disease, although a nonsignificant association with allergic-type conditions was observed. Studies are in progress to examine the modality of the defect's genetic transmission, as well as problems related to possible functional alterations and correlated clinical consequences.