An analysis of the computer searches for HLA-matched donors was done for 100 alloimmunized patients requiring HLA-matched platelet transfusion support. With a pool of 2470 donors, an average of only 1.3 (range 0-14) perfectly matched donors were potentially available per patient. Patients had an average of 9 donors with no mismatched antigens. Although the number of potential donors increased when cross reactive or single antigen mismatches were considered, our estimate of donor availability of this type is lower than reported previously. More donors were identified for the 39 patients with relatively common genotypes than for the remaining 61 patients. Of the latter group, 25 patients did not have any potential donors with "no mismatches" while in most of the others the only suitable donor was a family member. These data suggest that, given the limitations of platelet donor selection using current HLA typing technology, it may be difficult to provide long-term platelet transfusion support for alloimmunized patients with unusual HLA types. This fact should be considered when planning intensive therapeutic regimens for alloimmunized patients with leukemia.