The urinary proteins of seven patients with asymptomatic low molecular weight proteinuria (ALMP) were compared with those of Lowe syndrome and Fanconi syndrome. A characteristic electrophoretic pattern was seen in ALMP by isoelectric focusing. Two dimensional electrophoresis revealed that the ALMP proteins were localized in a more negatively charged region of lower molecular weight and consisted of a smaller variety of proteins in comparison with those from Fanconi syndrome and Lowe syndrome. These results suggest that ALMP might be an isolated form of other types of congenital renal tubular diseases from the viewpoint of protein reabsorption and has a lesser functional defect on the proximal tubules.