Objectives: IgG4-related disease (IgG4-RD) may present as ‘idiopathic’ retroperitoneal fibrosis (IRPF). We aimed to determine the occurrence of IgG4-retroperitoneal fibrosis (IgG4-RPF) in a nationwide study on patients with newly diagnosed IRPF, and to compare histopathological, imaging, and clinical features in the IgG4-RPF and non-IgG4-RPF subsets. Method: The National Danish Pathology Register was searched for biopsy codes relating to retroperitoneal tissue from 1 January 2004 to 31 December 2013. Secondary causes of RPF were excluded. Among 724 candidate cases, 68 were identified with IRPF. Clinical, laboratory, and imaging recordings were reviewed, and tissue blocks were scrutinized for IgG4-RPF features according to international consensus. Results: Forty-two patients (28 males), median age 56 (25–74) years were included. Nineteen (45%) met the criteria for IgG4-RPF, seven with definite and 12 with possible IgG4-RPF, while 23 had non-IgG4-RPF. Local manifestations and laboratory measures did not differ between RPF subsets. Arterial hypertension (p = 0.037) and periaortic fibrosis (p = 0.024) were more common in IgG4-RPF vs non-IgG4-RPF. Plasma cell IgG4/total IgG ratios ≥ 40% were associated more with core histopathological features of IgG4-RD compared to ratios < 40% (p < 0.001). There was a positive correlation between tissue IgG4-positive plasma cells and eosinophil cell count in patients with IgG4-RPF (rho = 0.50, p = 0.043). Conclusion: Forty-five per cent of this nationwide study population with newly diagnosed IRPF could be reclassified with IgG4-RPF. The association between high numbers of IgG4-bearing plasma cells and histopathological features of IgG4-RPF supports IgG4-bearing plasma cells with a perturbed distribution between IgG4 and total IgG being implicated in the pathogenesis of IgG4-RPF.