In the present study we assessed the prevalence of thin glomerular basement membrane (TGBM) in a large group of native kidney biopsies done for evaluation of renal disease. TGBM was present in 54 of 1078 biopsies (5%). In 12 of 54 biopsies (24%), TGBM was the only abnormality present. In the remaining biopsies TGBM was associated with other pathologic diagnoses. The overall prevalence of TGBM in this series is comparable to previous population studies. TGBM is significantly more common in patients with IgA nephropathy and mesangial proliferative glomerulonephritis. Compared to control patients, individuals with TGBM were more likely to have a history of familial hematuria (14% vs. 43%, P = 0.02). Furthermore, examination of urinary sediments in first degree relatives of patients with TGBM demonstrated microscopic hematuria in 92% of families and, in those families, hematuria was present in 47 +/- 6% of relatives. In contrast, hematuria was discovered in 38% of families of control patients, affecting 25 +/- 5% of relatives. In conclusion, the presence of TGBM in a kidney biopsy is highly predictable for the presence of familial microscopic hematuria, even in patients in whom TGBM is associated with another glomerulopathy. The present data also indicate that patients with TGBM nephropathy often have concomitant IgA nephropathy and mesangial proliferative glomerulonephritis.