BACKGROUND: Chronic plaque psoriasis can be sub-divided into two groups according to age of onset; type 1 (early-onset: before 40 years) and type 2 (late-onset: at or beyond 40 years). So far, 36 genetic loci have been associated with early-onset psoriasis in Caucasian genome-wide association studies, whilst few studies have investigated genetic susceptibility to late-onset psoriasis. OBJECTIVES: The aim of the current study was to characterise the genetics underpinning late-onset psoriasis. METHODS: We genotyped 543 late-onset psoriasis cases and 4,373 healthy controls using the Immunochip array; a dense genotyping chip containing single nucleotide polymorphisms previously associated with autoimmune diseases. Imputation using SNP2HLA and stepwise logistic regression analysis was performed for markers spanning the HLA gene region. RESULTS: Two loci (HLA-C and IL12B) previously associated with early-onset psoriasis showed significant association at genome-wide threshold in the current study (P