Review of the atopic diseases suggests a redefinition of the term "atopy" is indicated to reflect new information that has become available during the 60 years since the term was introduced. Atopy may be viewed as a manifestation of a still undefined defect. It is characterized by certain clinical findings and frequently by derangement of the immune and autonomic nervous systems. The atopic diseases are a group of seemingly unrelated conditions--eczema, asthma, rhinitis, hypertrophic sinusitis, and perhaps vernal conjunctivitis and migraine--which cluster in individuals and families. In the respiratory tract and eye, eosinophils in the tissues and secretions are characteristic and are not dependent on the presence of immediate hypersensitivity. Symptoms suggestive of basophil and mast cell mediator release are common to all the atopic diseases, and there is some evidence that nonimmunologic mediator release is enhanced in atopic patients. In the most clearly defined atopic diseases, eczema and asthma, approximately 80% of patients have an increased IgE response to normal environmental allergens. Accompanying and perhaps underlying these enhanced IgE responses are deficiencies of T cell numbers and function particularly in the suppressor T lymphocytes. Evidence exists that decreased beta-2-adrenergic and increased cholinergic and alpha-adrenergic responsiveness accompany and perhaps underlies the atopic diseases irrespective of the presence or absence of allergy.