This report presents serologic equivalents of human leukocyte antigen (HLA)-A, -B, -C, -DRB1, -DRB3, -DRB4, -DRB5, and -DQB1 alleles. The dictionary is an update of the one published in 2001. The data summarize equivalents obtained by the World Health Organization (WHO) Nomenclature Committee for Factors of the HLA System, the International Cell Exchange (UCLA), the National Marrow Donor Program, recent publications, and individual laboratories. This latest update of the dictionary is enhanced by the inclusion of results from studies performed during the 13th International Histocompatibility Workshop and from neural network analyses. A summary of the data as recommended serologic equivalents is presented as expert assigned types. The tables include remarks for alleles, which are or may be 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. The serological-DNA equivalent dictionary will also aid in typing and matching procedures for organ transplant programs whose waiting lists of potential donors and recipients are comprised of mixtures of serologic and DNA-based typings. The tables with HLA equivalents and a questionnaire for submission of serologic reaction patterns for poorly identified allelic products will be made available through the World Marrow Donor Association Web page (www.worldmarrow.org).