Idiopathic hypereosinophilic syndrome (IHES) is a rare condition of uncertain etiology characterized by marked peripheral blood eosinophilia and organ system dysfunction that cannot be explained by any factor other than the presence of eosinophils or their potentially toxic products. Diagnostic criteria include 1) a sustained eosinophilia greater than 1500/mm for longer than 6 months, 2) absence of other causes of eosinophilia, including parasitic infections and allergic diseases, and 3) multiorgan involvement (ie, lungs, heart, central nervous system, skin, liver, joints). Steroids represent the initial therapeutic approach, although for those patients unresponsive to steroids, cytotoxic chemotherapy should be considered. We describe a case of IHES characterized by polyarticular inflammatory joint involvement, hypereosinophilia, and urticarioid skin manifestations without visceral involvement. Synovial fluid smears as well as pathology of skin lesions and knee synovial membrane confirmed the presence of numerous eosinophils. The patient was successfully treated with a combination therapy of a cysteinyl leukotriene receptor antagonist (montelukast) and ketotifen. Hypereosinophilic syndrome like in this patient is a rare cause of polyarthritis.