The frequencies of 29 HLA-DRB1*04 alleles were determined for five major U.S. populations found within a hematopoietic stem cell volunteer donor registry. One hundred sixty-one DRB1*04 positive individuals from each of the self-described groups, Caucasians, African-Americans, Asian/Pacific Islanders, Hispanics, and Native Americans, were randomly chosen from a database of 82,979 unrelated persons. Subjected to polymerase chain reaction-sequence-specific oligonucleotide probe (PCR-SSOP) typing, these 805 individuals carried a total of ten different DRB1*04 alleles, ranging from DRB1*0401 to DRB1*0411 with DRB1*0409 conspicuously absent from all five groups. The distribution of DRB1*04 alleles varied among the groups, with DRB1*0401 being predominant in Caucasians, African-Americans, and Native Americans. DRB1*0404 and DRB1*0407 were the two most commonly observed alleles in Hispanics, whereas DRB1*0405 and DRB1*04031 were most common in Asian/Pacific Islanders. The remaining 18 DRB1*04 alleles known at the time of the study were not observed. Although not observed in the frequency study, seven previously unreported DRB1*04 alleles are also described.