Localized Castleman's disease (CD) has been divided two types, the classical hyaline vascular (HV) type and the rare plasma cell (PC) type. Recently, we have reported two cases of IgG4-related disorder of the retroperitoneum showing PC type of CD. To further clarify the clinicopathological findings of CD of the retroperitoneum, eight such cases have been studied. A single lesion was located in the retroperitoneum (n=3), ureter (n=2) and renal hilum (n=2). One case had bilateral ureter lesions. The HV type of CD accounts for approximately 90% of cases. However, 50% (n=4) of our cases were the PC type of CD. Three of the four lesions of HV type had lymph node lesions, whereas all four PC type of CD were soft tissue masses. These clinicopathologic findings appear quite different from previous descriptions. Immunohistochemical study demonstrated numerous IgG4(+) plasma cells accounting for more that 50% of IgG4(+) cells in three cases of the four PC type of CD. Moreover, serum IgG4 concentration was increased in two of the four cases of PC type of CD that were examined. The serum interleukin-6 levels were within the normal range in two cases of PC type that were examined. The present study suggests that a majority of the PC type of CD arising in the retroperitoneum appears to be an IgG4-related disorder.