Many patients in need of a hematopoietic stem cell transplant do not find a fully matching donor although more than 11,000,000 potential donors are registered worldwide. Therefore, it is relevant to recruit donors who add diversity to the donor pool. We present the 'Roots' approach that includes the selection of already registered donors with rare HLA phenotypes and the recruitment of relatives of these donors. Two projects (Roots A and B) with different donor selection criteria were carried out. HLA phenotype frequency distributions of new donors differ significantly from the respective control groups: 2.7% of Roots A donors versus 1.1% of the control group have an HLA-AB phenotype that is unique in the DKMS file (P=0.001). Additionally, 39.5% of Roots B donors but only 18.3% of the control group have a unique HLA-ABDR phenotype (P<0.001). Similar results are found when phenotypes that are at most available 10 times in the DKMS donor file are analyzed. The results show that the Roots approach is generally suited to increasing the ratio of donors with rare HLA phenotypes in a donor file. Additional costs of Roots donor recruitment seem justified through the ratio of recruited donors with rare HLA phenotypes.