IgA-bearing peripheral blood lymphocytes, serum IgA, urinary sediments and HLA types of patients with IgA nephropathy and members of their families were examined to elucidate whether some familial factors might be related to the development of IgA nephropathy. Ten patients with IgA nephropathy, 31 family members and 36 age-matched healthy persons were examined. All families included certain members with increased amounts of IgA-bearing peripheral blood lymphocytes. The pattern of the emergence of family members with increased IgA-bearing lymphocytes was vertical. Some family members who had increased IgA-bearing lymphocytes showed microhematuria at the time of the study. There was no significant correlation between the amounts of IgA-bearing peripheral blood lymphocytes and levels of serum IgA. HLA types of the ten patients did not show significant deviation from those in the general population. It is suggested that the measurement of IgA-bearing peripheral blood lymphocytes among family members is useful for the screening of patients with IgA nephropathy.