Marked plasmacytosis in the bone marrow is a rare finding in young people. If present it is secondary to an underlying disease condition. Castleman disease is a rare form of lymphoproliferative disorder. Due to its rarity, it is not frequently considered in the differential diagnosis of lymphadenopathy. As in any other lymphoproliferative disorder Castleman disease can also be associated with various immune mediated cytopenia. This case report is on a teenager with persistent lymphadenopathy of whom the presence of marked bone marrow plasmacytosis led to the diagnosis of plasma cell type Castleman disease which went into complete remission with single agent vincristine and radiotherapy with subsequent severe immune thrombocytopenia during follow up.