We examined kidney biopsy specimens obtained from 40 adult patients with isolated hematuria to determine the renal pathology and the incidence of thin glomerular basement membrane nephropathy (TGBMN). Light microscopy showed minor glomerular abnormalities in 26 patients (65%), focal and segmental lesions in 3 patients (8%), and mild diffuse proliferative glomerulonephritis in 11 patients (28%). Immunofluorescence microscopy showed IgA nephropathy (IgA-N) in 16 patients (40%), in whom no progressive lesions were identified. We measured the glomerular basement membrane (GBM) thickness using electron microscopy, and TGBMN was identified in 4 patients (10%). Our results suggest that IgA is a major pathological finding in adult patients with isolated hematuria. GBM thinning does not appear to be a major cause of glomerular hematuria.