Retroperitoneal fibrosis (RF) is a rare disease characterized by inflammation and fibrosis of retroperitoneal soft tissues. It is classified into two types: idiopathic (iRF) and secondary (sRF). The aim of the study was to investigate the relationship between iRF and IgG4-related disease (IgG4-RD) and to eventually extend the clinicopathological features of this condition by analysis of the sample comprising six iRF and six sRF patients. The iRF patients included four males and two females, aged 12-62 years (median 55 years). Two lesions were periaortic, one was periureteral, and three cases showed both periaortic and periureteral localization. Two patients had increased serum levels of IgG4. None of the patients developed any manifestation of IgG4-RD during the follow-up period ranging for 15-133 months (median 43 months). Microscopically, in two iRF cases, fibrosis was highly cellular encircling the vessels, nerves, and paraganglia. Phlebitis was found in all cases and being obliterative in four. Lymphocytic inflammation with formation of follicles and plasma cell infiltration were scored as severe in five iRF cases. The numbers of IgG-positive plasma cells ranged 0-373 per 1 HPF (high power field; median 132) and of IgG4-positive plasma cells 0-238 per 1 HPF (median 91). The IgG4/IgG ratio values ranged 0.38-0.74 (median 0.68). Two of the iRF cases were diagnosed as definite and three cases as probable IgG4-RD. To the contrary, none of the sRF cases met the diagnostic criteria for either definite, probable, or possible IgG4-related disease. Our results indicate that a substantial portion of iRF cases, including some of very rare pediatric cases, is a manifestation of IgG4-RD.