Two related male patients with mesangiocapillary glomerulonephritis (MCGN) are described demonstrated by renal biopsy, inherited as an X-linked disorder. Family investigations failed to reveal any underlying immunological defects or a marker for the female carrier state. The age at diagnosis, the result of discovery of proteinuria on routine urine testing during infancy, is earlier than in any other reported cases of MCGN. This raises the possibility that this variety of MCGN may develop in utero and be detectable by alpha-fetoprotein maternal screening.