This report presents the serologic equivalents of 123 HLA-A, 272 HLA-B and 155 HLA-DRB1 alleles. The equivalents cover over 64% of the presently identified HLA-A, -B and -DRB1 alleles. The dictionary is an update of the one published in 1999 and also includes equivalents for HLA-C, DRB3, DRB4, DRB5 and DQB1 alleles. The data summarize information obtained by the WHO Nomenclature Committee for Factors of the HLA System, the International Cell Exchange (UCLA), the National Marrow Donor Program (NMDP) and individual laboratories. In addition, a listing is provided of alleles which are expressed as antigens with serologic reaction patterns that differ from the well-established HLA specificities. The equivalents provided will be useful in guiding searches for unrelated hematopoietic stem cell donors in which patients and/or potential donors are typed by either serology or DNA-based methods. These equivalents will also serve typing and matching procedures for organ transplant programs where HLA typings from donors and from recipients on waiting lists represent mixtures of serologic and molecular typings. The tables with HLA equivalents and a questionnaire for submission of serologic reaction patterns for poorly identified allelic products will also be available on the WMDA web page: http://www.worldmarrow.org.