One hundred patients diagnosed with hypocomplementaemic MPGN (C3 < 40%) were studied to determine the presence of C3 nephritic factor (C3NeF) and/or C4 nephritic factor (C4NeF). Of those studied, 12 were C3NeF-positive, nine were C4NeF-positive and 10 were positive for both C3NeF and C4NeF. In the 10 patients both C3NeF- and C4NeF-positive, a marked decrease in C3 and C5 levels and a decrease in levels of late components from C6 to C9 were observed. This observation was in contrast to that seen in patients who were either C3NeF- or C4NeF-positive. Patients positive for both C3NeF and C4NeF continued to exhibit hypocomplementaemia after therapy. Immunofluorescent findings revealed heavy C3 immunoglobulin deposits in the 10 patients who were both C3NeF- and C4NeF-positive, whereas no such deposits were found in those patients who were either C3NeF- or C4NeF-positive only. When those patients who were both C3NeF- and C4NeF-positive were compared with those who were either C3NeF- or C4NeF-positive, nephritic syndrome and a poor prognosis were observed more frequently. This study demonstrates a correlation between clinical outcome and hypocomplementaemic MPGN. Further investigations of MPGN as an autoimmune disease are necessary.