Allele and genotype frequencies at the HLA-DQ alpha locus were determined by the use of polymerase chain reaction (PCR) and the reverse dot blot format. 6 HLA-DQ alpha alleles and 21 genotypes were detected among the 178 unrelated individuals from Barcelona. A fair agreement was found between observed and expected numbers assuming a Hardy-Weinberg equilibrium. The Barcelona population is statistically similar to the Finnish population, but differs significantly from the Japanese, Black and Caucasoid series.