ALFRED (http://alfred.med.yale.edu) is a free, web accessible, curated compilation of allele frequency data on DNA sequence polymorphisms in anthropologically defined human populations. Currently, ALFRED has allele frequency tables on over 663,400 polymorphic sites; 170 of them have frequency tables for more than 100 different population samples. In ALFRED, a population may have multiple samples with each 'sample' consisting of many individuals on which an allele frequency is based. There are 3566 population samples from 710 different populations with allele frequency tables on at least one polymorphism. Fifty of those population samples have allele frequency data for over 650,000 polymorphisms. Records also have active links to relevant resources (dbSNP, PharmGKB, OMIM, Ethnologue, etc.). The flexible search options and data display and download capabilities available through the web interface allow easy access to the large quantity of high-quality data in ALFRED.