We analysed the allelic frequencies of class II human leucocyte antigen (HLA)-DRB1, DQA1, DQB1 and DPB1 by polymerase chain reaction/sequence-specific oligonucleotide probe hybridization typing in 76 Taiwanese psoriasis vulgaris (PSV) patients and 238 Taiwanese non-psoriatic controls. The analysis revealed the following: (i) the DRB1*0701 allele was positively associated with PSV (relative risk, RR = 6.4, corrected P-value, Pc < or = 0.001); (ii) the DRB1*1401 allele was positively associated with type I PSV (age at onset < 40 years) (RR = 3.5, Pc < or = 0.001); (iii) the DQA1*0501 allele was negatively associated with PSV (RR = 0.4, Pc < or = 0.001); (iv) there was no significant association of HLA-DP genes with PSV; and (v) there was a strong association of beta-chain phenylalanine at position 37 (Phe 37) and glutamate or glutamine at position 74 (Glu 74/Gln 74) with PSV (RR = 3.5, Pc < or = 0.001 for the association of Phe 37 with PSV: RR = 2.2, Pc < or = 0.001 for the association of Glu 74/Gln 74 with PSV). The positive association between PSV and the DRB1*0701 allele is consistent with previous reports. The negative association of the DQA1*0501 allele is reported only in Finland, whereas the positive association between PSV and the DRB1*1401 allele has never been described before. Trans-racial studies may shed further light on the association of class II HLA alleles or other closely linked genes with the development of PSV. Phe 37 (a large, non-polar amino acid) and Glu 74/Gln 74 (both negatively charged amino acids) were the polymorphic residues in pockets 9 and 4, respectively, of the beta-chain, which may have increased their affinity for the small non-polar amino acids and basic amino acids of the psoriatic antigen peptide, thereby activating the T lymphocytes. This finding may facilitate the identification of a psoriatic antigen.