A 49-year-old woman was admitted to our hospital because of fever of unknown origin. The patient had long-lasting spiking fever, hepatosplenomegaly, pleural effusion, and skin rash. Laboratory tests showed marked leukocytosis and an extremely high serum ferritin level (240 000 ng/ml) accompanied by disseminated intravascular coagulation and hemophagocytic syndrome. Most of the patient's features were compatible with a diagnosis of adult-onset Still's disease (AOSD), the rash, however, was not a typical rheumatoid rash but multiforme erythema. Biopsy of a breast nodule revealed breast cancer, leading us to a diagnosis of paraneoplastic syndrome mimicking AOSD. Although this is a rare disorder, cases resembling the present one have been reported, indicating the importance of including paraneoplastic syndrome in the differential diagnosis of AOSD.