Although matching of kidney donors and recipients for antigens of the HLA-DR series is considered to have a significant influence on graft outcome, the independent effects of incompatibilities for the closely linked HLA-DQ series of antigens are unknown largely because serological compatibility for HLA-DR usually ensures a corresponding compatibility for HLA-DQ. In a retrospective study using restriction fragment analysis we have identified clear HLA-DQ mismatches in 25 out of 62 recipients of HLA-DR-compatible kidney grafts. Though these incompatibilities correspond to differences at the cell surface and hence potential alloantigenic stimuli, none of the eight clinical parameters examined indicate that HLA-DQ mismatches in the presence of compatibility for HLA-DR have any significant effect on either graft function or outcome.