We report a case of malignant histiocytosis which began with the skin lesions of panniculitis. A 32-year-old woman presented with recurrent erythematous plaques, subcutaneous nodules, and ulcers on the trunk and the extremities and intermittent fever for 7 months. The cutaneous lesions consisted of erythematous and brownish irregular-shaped patches and tender cutaneous nodules 0.5-1.0 cm in diameter. Central necrosis and shallow ulcers were seen in the lesions. The patient also suffered from general fatigue, arthralgia, and weight loss. She was anemic and thrombocytopenic and had progressive impairment of liver function with coagulation defect. Histopathological study of skin lesions showed lobular panniculitis without vasculitis in the subcutaneous fat tissue. In the panniculitis lesion, moderate mixed cell infiltration consisting of lymphocytes and histiocytes was observed. Bone marrow aspiration revealed an increase in the number of histiocytes, mostly immature with active phagocytosis of erythroid cells, myeloid cells, and platelets. She was diagnosed as having malignant histiocytosis and treated with cyclophosphamide, vincristine, and prednisolone which she responded well; her fever subsided and the lesions healed with hyperpigmentation. In this patient, benign histiocytes with hemophagocytosis without immature forms were found in the skin lesions. According to our knowledge, this is the first Thai report of malignant histiocytosis with clinical features of panniculitis.