Eosinophils normally account for 1–5% of leucocytes in peripheral blood in adults. The reference range in absolute numbers is 0.04–0.4 × 109/l. In infants less than one year old, the eosinophil count is higher and their proportion may be up to 10% of leucocytes (2.4 × 109/l). An eosinophil count of > 0.4 × 109/l in patients over one year of age is regarded as eosinophilia. Consider the following: Could the eosinophilia be a sign of some underlying disease, which needs medical attention (based on history and status)? Can the eosinophilia itself pose a threat to the patient (eosinophils can cause tissue damage by secreting proinflammatory cytokines)? There is diurnal variation in the blood eosinophil count. It has an inverse relationship with blood glucocorticoid levels. The blood eosinophil count is highest in the evening and lowest in the morning. Eosinophil levels are slightly higher during menstruation. Physical strain temporarily increases the blood eosinophil count, but psychological stress and beta-blockers may decrease it.