Patients 1 to 100 years of age with eosinophil counts greater than 750/ml or an abnormal accumulation of eosinophils in the skin or body tissues may be eligible for this study. All participants will have a thorough medical history, physical examination and blood tests. Depending on the person's age and symptoms, other diagnostic tests may be done, including specialized studies of the eye, lungs, skin, bone marrow, nerves or heart. This is not a treatment study, and no experimental treatments will be offered. Patients who require treatment will receive standard medical care.