The thin basement membrane lesion is the underlying morphologic change in most families with benign familial hematuria and may also be seen in sporadic cases of hematuria. Many of these kindreds are carriers of autosomal recessive Alport syndrome. Patients show persistent microscopic hematuria, often with intermittent macroscopic hematuria. Of patients biopsied for persistent isolated hematuria, thin basement membranes may be present in 20% to 25%. Prognosis is generally benign, with slight increased risk of chronic kidney disease in carriers of Alport syndrome.