Background:Idiopathic hypereosinophilic syndrome (HES) is a rare and potentially lethal disorder characterized by persistently elevated eosinophil counts without any underlying causes. Two variants, the myeloproliferative and lymphocytic hypereosinophilic syndrome, have been identified. The symptoms are variable and related to the organs involved (cardiovascular system, skin, central and peripheral nervous system, gastrointestinal tract, eyes). Skin lesions can be the dominating and/or presenting symptom in about 50% of patients.Main observations:We describe a 54-year-old man with a 12-year history of skin lesions, clinically consistent with psoriasis and psoriatic erythroderma. The patient was treated with methotrexate with no response. He experienced intense pruritus, dry/coarse skin and palmoplantar hyperkeratosis. Histopathology showed spongiotic dermatitis with no epidermotropism. Inflammatory infiltrates in upper dermis consisted predominantly of lymphocytes and eosinophils. Peripheral and tissue eosinophilia, immunophenotyping, and results of FIP1L1-PDGFRA gene analysis were suggestive of lymphocytic HES. The patient was treated with hydroxycarbamide (1 g/day), prednisolone (40 mg/day) and antihistamines with improvement. Conclusions:HES requires early treatment to prevent severe damage of targeted organs. The pleomorphic dermatological manifestations may delay the diagnosis. This case shows the importance of wide differential diagnosis of erythroderma. In this article we discuss the diagnostic criteria, the recommended work-up and management of idiopathic hypereosinophilic syndrome variants.