Of 188 cases of Kaposi's sarcoma arising de novo after transplantation, HLA-A, -B typing was available for 135 and HLA-DR typing available for 67. Compared to the reported HLA phenotype frequencies of renal transplant recipients in the Southeast Organ Procurement Foundation (SEOPF), there is a significantly decreased frequency of HLA-A1 and HLA-B7, and increased frequency of HLA-B5, -B8, -B18, and -DR5. The most striking characteristic of the Kaposi's sarcoma group was its ethnic background. Fifty-six percent of patients were Italian, Greek, Jewish, or Arabic. When this ethnic background is considered, the expected HLA phenotype frequencies are almost exactly the same as in the Kaposi's sarcoma population. The quality of donor-recipient HLA match was evaluable for 106 patients. Only 22% had four or more mismatches, and 59% had at least two antigens matched. This argues against poor donor-recipient matching as a risk factor for developing Kaposi's sarcoma after transplantation.