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An 11-year-old boy with Klinefelter syndrome had Castleman disease (CD) of plasma cell type develop. Nonregulated antibody production mimicked systemic lupus erythematosus (SLE). Hepatitis C virus (HCV) infection caused significant disease worsening. The patient was treated with a daily dosage of 2 million units/m2 of IFN-α. Dramatic clinical improvement and decreasing autoimmune phenomenon were observed. HCV RNA were cleared. Hypergammaglobulinemia did not change. The boy has been living for 8 years with his disease. Plasma cell type CD can mimic collagenosis. Disease worsening is caused by HCV, though it can be reversed with IFN-α. Klinefelter syndrome may be a genetic susceptibility factor for CD in some cases.