We describe here an online Y-chromosomal short tandem repeat haplotype reference database (YHRD) for U.S. populations, which represents 9-locus Y-STR haplotypes for 1705 African-Americans, European-Americans and Hispanics as of October 2001. This database is available online (http://www.ystr. org/usa/), free to access and was generated in order to supply the U.S. forensic DNA community with a valuable resource for frequencies of complete or incomplete 9-locus Y-STR haplotypes, as well as information about typing protocols and population genetic analyses. Pairwise R(ST)-statistics derived from the Y-STR haplotypes indicate no significant substructure among African-American populations from different regions of the U.S., nor (usually) among European-American and Hispanic populations. Thus, pooling of Y-STR haplotype data from regional populations within these three major groups is appropriate in order to obtain larger sample sizes. However, pooling of different major populations is generally not recommended due to statistically significant differences between African-American populations and all European-American/Hispanic populations, as well as between some European-American and Hispanic populations.