Eosinophilia is occasionally a feature of rheumatic disease. The differential diagnosis of eosinophilia includes parasitic infection, systemic vasculitides, eosinophilic arthritis, and myopathies, together with the idiopathic hypereosinophilic syndrome and malignancy. Careful evaluation of the patient should enable an accurate diagnosis to be made. Parasitic infection is the commonest cause of eosinophilia worldwide and can cause systemic disease, as illustrated by the report ofSarcocystismyositis in a group of military personnel in Malaysia. A persistent arthropathy associated with eosinophilia, but not with parasitic infection, has been reported from the far East. Drugs may also cause eosinophilia, and there has recently been much discussion of the relation between Churg-Strauss syndrome and the leukotriene antagonist zafirlukast. The present view is that reduction of steroid dose allows unmasking of previously undiagnosed Churg-Strauss syndrome. The idiopathic hypereosinophilic syndrome may represent a lymphoproliferative process; evidence for this comes from the demonstration that many patients have a clonally expanded population of aberrant T cells.