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Background Psoriasis is a heterogeneous disease in its clinical expression. Both genetic and environmental factors are thought to contribute to the pathogenesis of the inflammatory and hyperproliferative components of the typical skin lesions. Predisposing genetic influences include associations with human leucocyte antigens (HLA) of which that with HLA-Cw6 is the strongest. Guttate psoriasis is a specific clinical manifestation of psoriasis frequently associated with group A β-haemolytic streptococcal throat infection. Objectives We set out to determine whether further clinical subdivision of psoriasis is associated with tighter correlation with HLA-C alleles. Patients/methods We determined the HLA-C locus genotype of 29 caucasian patients with guttate psoriasis presenting consecutively with guttate psoriasis associated with a history of a sore throat and/or an antistreptolysin O titre > 200 IU mL-1. Polymerase chain reaction typing using sequence-specific primers was used to detect all known HLA-C alleles. These data were compared with a control population of 604 random caucasian cadaver donors. Results All patients (100%) with guttate psoriasis carried the Cw*0602 allele compared with 20% of the control population (odds ratio = ∞; 95% confidence limits 25·00–∞; Pcorrected < 0·0000002). Conclusions This result is consistent with HLA-Cw*0602 playing a part directly in the pathogenesis of guttate psoriasis. [ABSTRACT FROM AUTHOR]