Although rare, renal involvement during hypereosinophilic syndromes can lead to life-threatening situations. Since eosinophilic renal lesions can occur in a wide range of primary or secondary diseases, diagnosis can pose difficult clinical dilemmas. In some settings, renal lesions may be a predictable complication as in essential hypereosinophilic syndrome or angiolymphoid hyperplasia with eosinophilia. In other cases, renal lesions may be a highly unusual event secondary to cholesterol embolization, drug-induced reactions, immunoallergic responses, eosinophilic helminthic infection, or maintenance hemodialysis. The mechanisms of renal involvement are complex. In hypereosinophilic syndromes, renal involvement has been attributed to the deleterious effects of eosinophil granules and possibly to micro-emboli from the heart in patients presenting fibroplastic endocarditis or eosinophilic myocarditis. Most secondary forms are usually due to an immuno-allergic process leading to deposit of immune complexes in glomeruli. The effects of polynuclear eosinophils could also be due to release of cytokines and other mediators such as leukotriens. Cholesterol embolization involves a different mechanism in which hypereosinophilia is often moderate and accessory to arteriolar lesions. Eosinophiluria may be observed in any setting but the prognostic value of this finding as well as the mechanism underlying remain unclear.