Summary  Background Psoriasis is strongly associated with certain human leucocyte‐associated antigens, especially HLA‐Cw*0602. Patients who are HLA‐Cw*0602 positive have been reported to have more active disease and a younger age at disease onset than HLA‐Cw6‐negative patients.

Objectives To ascertain whether there are differences in the clinical features and relative risk between HLA‐Cw*0602 homozygous and heterozygous psoriasis patients.

Methods One thousand and six patients with chronic plaque psoriasis were evaluated clinically and HLA‐C typed. In addition, 512 unrelated controls were typed for HLA‐C.

Results Of the patients 646 (64·2%) were HLA‐Cw*0602 positive, and 68 (6·8%) were homozygous for this allele. Heterozygosity was associated with a relative risk of developing psoriasis of 8·9 compared with 23·1 for the Cw6 homozygous patients. The homozygous patients also had an earlier disease onset (mean 15·0 vs. 17·8 years, P = 0·04). However, the Cw6 homozygotes did not differ from the heterozygotes with respect to disease severity, guttate onset, distribution of plaques, nail changes or any other clinical parameter recorded.

Conclusions Homozygosity for the gene in the major histocompatibility complex region has a major additive impact on the risk of developing psoriasis and predisposes to an earlier disease onset, but does not have any marked influence on the phenotype or the severity of the disease.