A total of nineteen children were studied because of asymptomatic hematuria. all had normal renal function, and immunologic and urologic studies. Renal tissue was obtained by percutaneous biopsy and examined by light, immunofluorescence and electron microscopy. On electron microscopy, thickness of the glomerular basement membrane (BGM) was found to distinguish benign familial hematuria (BFH - 10 cases) from non familial idiopathic recurrent hematuria (IRH - 9 cases). Measurements were taken in areas of peripheral glomerular capillary loops (minimum of five) where both cell membranes could be resolved to insure tht determinations were uniform and represented ideal cross sections. The mean thickness +/- SE of GBM in IRH of 303.3 +/- 16.9 nM was greater than that noted in BFH of 207.1 +/- 11.99 nM (P less than .002). The mean thickness +/- SE of the lamina densa in IRH of 236 +/- 15.36 nM was significantly greater than that observed in BFH of 128.5 +/- 11.7 nM (P less than .001). In both BFH and IRH the GBM and LD were uniform in thickness (CV = 20%) and qualitatively normal. The uniform attenuation observed in BFH can be used to distinguish this condition from IRH.