A 45-year-old white woman was found to have microscopic hematuria during her annual physical examination. After a negative urologic workup, she returned 5 months later with nephrotic syndrome, renal insufficiency, and hypocomplementemia. Renal biopsy showed a nodular sclerosing glomerulopathy that could not be further characterized because of inadequate tissue for immunofluorescence. The patient returned 8 months later with chronic renal failure. A repeat renal biopsy showed deposits composed of immunoglobulin G (IgG) heavy chain and complement components C3 and C1 along glomerular, tubular, and vascular basement membranes, with negativity for kappa and lambda light chains, findings consistent with heavy chain deposition disease (HCDD). The heavy chain subclass was exclusively IgG3. Staining with monoclonal antibodies to epitopes of the constant domains of IgG heavy chain showed a CH1 deletion, indicating a truncated heavy chain. On review of the previously reported cases of HCDD, common clinical presentations include nephrotic syndrome, renal insufficiency, hematuria, and, in some cases, hypocomplementemia. In most patients, the hematologic disorder is mild, without overt myeloma. Light microscopy shows a nodular sclerosing glomerulopathy, and heavy chain deposits are detectable within basement membranes throughout the kidney by immunofluorescence and electron microscopy. There is no effective treatment for this condition, and virtually all patients progress to chronic renal failure.