A group of 9 children with persistent isolated hematuria in the absence of other evidence of renal disease is presented. The children represent about one third of our patients with benign recurrent hematuria and are unique in that their C4s, in a functional assay, are lower than one would expect from their immunochemical level, resulting in an abnormally low C4 hemolytic efficiency. This feature occurs in the absence of any other evidence of complement dysfunction. Similar abnormalities were not seen in patients with other hypocomplementemic renal disease or congenital C4 deficiency. The patients’ C4 molecules were identical to normal in charge, mobility, antigenic configuration, and ability to participate in an immune reaction. All patients had at least one C4 null gene on genetic analysis and 3 had double null genes. A similar pattern of C4 abnormality was not found in other patients with hypocomplementemic renal disease.