We have studied the influence of serological matching for ten HLA-DR antigens on the occurrence of acute cellular rejection in heart transplantation by correlating the findings in routine endomyocardial biopsies taken during the first posttransplant year with the results of HLA typing of all recipients of a first cardiac graft and their donors during 1983-1994 at our center. We found that recipients of HLA-DR matched hearts had a lower frequency of acute cellular rejection, especially so for the moderate/severe rejection grades. Also, rejection appeared earlier in the DR-mismatched combinations. Whether the mismatch was for one or two DR antigens did not make a significant difference, neither could we demonstrate any influence of HLA-A or -B mismatches. The survival of DR-matched cardiac grafts tended to be higher at 1 year than DR-mismatched grafts, but the difference did not reach statistical significance.