Thin-glomerular-basement-membrane (TGBM) nephropathy is among the most common causes of isolated hematuria. This autosomal dominant disorder is characterized by diffuse thinning of the GBM and is diagnosed by electron microscopic examination of renal biopsy tissue. A study of an affected kindred has revealed a mutation in the alpha chain of type IV collagen, resulting in abnormal basement membrane synthesis. Although the exact prevalence and prognosis is unclear, TGBM is usually regarded as a benign cause of hematuria and not associated with any untoward effect on renal function. We report a case of TGBM nephropathy, with associated proteinuria and progressive renal insufficiency. Other studies similarly contend that TGBM nephropathy may not be so benign. On the basis of these findings, we suggest that in some patients with TGBM nephropathy, progressive renal insufficiency may develop. We recommend a more vigilant approach in patients with TGBM nephropathy, especially if proteinuria is present.