Introduction. Immunoglobulin G4-related disease (IgG4-RD) is a rare immunemediated condition with expansion of plasmablasts/plasma cells and possible upregulation of DNAJB9. DNAJB9 (a co-chaperone protein that governs the integrity and functions of cells) was described as a specific marker for fibrillary glomerulonephritis, a rare proliferative form of glomerulonephritis displaying histological similarities with IgG4-related kidney disease. DNAJB9 may be a putative autoantigen. We aimed to study the immunohistochemical expression of DNAJB9 in IgG4-RD to explore its role in disease pathophysiology. Methods. We selected patients with definite IgG4-RD (RCD, ACR/EULAR or ICDC classification criteria), non-auto-immune (NAI) controls and Sjögren’s syndrome controls, and produced tissue slides for anti-DNAJB9 immunohistochemistry on which we performed double reading and applied our intensity scoring system ranging from 0 to 4. Our positive control was a renal sample suggestive of fibrillary glomerulonephritis. Results. We studied the tissue slides of 9 IgG4-RD patients and 15 NAI controls (including salivary gland, pancreatic, pulmonary, pleural, and retroperitoneal fibrotic tissue). Median intensity for endothelial cells was 0 in IgG4-RD and 2 in NAI controls (p<0.01). In each group, it was 3 for ductal epithelial cells (p=0.60), 2 for glandular epithelial cells (p=0.26) and 3 for inflammatory cells (p=0.74). All samples displayed a cytoplasmic and granular immunohistochemical staining pattern, with no difference between the 2 groups. Conclusions. There was no difference in terms of staining intensity for DNAJB9 between IgG4-RD patients and NAI controls regarding ductal and glandular epithelial cells and inflammatory cells. Surprisingly, endothelial cells showed significantly weaker staining in IgG4-RD, suggesting loss of DNAJB9 expression in those cells. The underlying mechanisms are unclear but the endostatin and regulatory T cell pathways could be partially explanatory and raise interest for further exploration.