To clarify the incidence of thin basement membrane disease (TMD) among the patients with idiopathic asymptomatic hematuria and/or proteinuria, in 217 serious renal biopsies (children 85, adults 132) were studied with clinical and morphometric analysis. TMD used is defined as follows: 1) Glomerulus in minor abnormalities; 2) GBM less than or equal to 200 nm in width, with more than 20% in total glomerular capillary surface; 3) Absence of significant immunoglobulins or complement components. Out of 217 patients 93% had either IgA nephropathy (55%), normal glomeruli (21%) or TMD (17%). TMD consisted of 22% in children and 14% in adults. Remained 15 consisted of non-IgA mesangial proliferative glomerulonephritis (6 cases), incomplete foot process disease (5), membranous nephropathy (2), membranoproliferative glomerulonephritis (1), and unclassified (1). Patients with TMD were found mostly (71%) in younger age less than 20 years old. Out of the patients with TMD, 38% had renal abnormality in the family history, but remainders were sporadic. The common urinary abnormality in TMD was microscopic hematuria occasionally with mild proteinuria (95%), while proteinuria only was rare (5%). Outcome of TMD was favorable prognosis with normal renal function. TMD was histologically divided into 2 groups; diffuse type (GBM thinning was more than 50% in the capillary surface) (19 cases; 51%), and focal type (it was less than 50%) (18 cases; 49%). The incidence of those family history was 64% and 15%, respectively (p less than 0.05). It was concluded that TMD was a popular disorder in patients with asymptomatic hematuria and/or proteinuria and it may expect 17% in incidence.