Results: HCDD was diagnosed in kidney biopsies of 4 women (mean age 73.0 years). Light microscopy showed diffuse nodular glomerulosclerosis (4/4), associated with mesangial proliferation (3/4) and capillary aneurysms (4/ 4). Immunofluorescence showed abundant mesangial deposits and ribbon-like deposits along glomerular, capsular, tubular and vascular basement membranes, positive for heavy chain IgG3 (3/4) and IgG1 (1/4), with deleted gamma CH1 domain (4/4). Complement C3 and C1q stained positive in all cases. By electron microscopy, punctate and powdery electron-dense deposits were found on the same locations. Conclusion: Immunofluorescence examination of kidney biopsies, including testing for immunoglobulin heavy and light chains, is crucial for diagnosis of HCDD. Our study confirmed that HCDD is peculiar among monoclonal immunoglobulin deposition diseases not only because of its rarity but also because of uniform histomorphologic pattern of nodular glomerulosclerosis with pronounced capillary aneurysms and significant proliferation due to complement activation. Deletion of the heavy chain CH1 domain and its significance in the pathogenesis has to be emphasized.