The current consensus on Castleman disease is that it is a group of several distinct lymphoproliferative disorders with different underlying pathogenesis and clinical outcomes. In 1980, Mori et al. proposed the concept of idiopathic plasmacytic lymphadenopathy with polyclonal hyperimmunoglobulinemia (IPL), a disease of unknown etiology, characterized by severe polyclonal hypergammaglobulinemia and generalized superficial lymphadenopathy. After Frizzera et al.'s landmark report in 1983, the term multicentric Castleman disease (MCD) gradually became established, and for a time, IPL was regarded as identical to MCD. However, with the subsequent recognition of human herpesvirus 8 (HHV8)-related MCD in the 1990s and the contributions by Kojima et al. in the 2000s, in which non-HHV8-related MCD (now called idiopathic MCD) was at least subclassified into IPL and others (non-IPL), it is now clear that the original distinctiveness of IPL is still maintained in MCD, which is a diverse collection of diseases.