Polymorphisms of the IL-10 promoter have been implicated in the genetic susceptibility to many autoimmune diseases, including psoriasis. Four putative functional single-nucleotide polymorphisms (SNPs) within the interleukin-10 promoter region (-3575T/A, -2763A/C, -1082G/A and -592C/A) were analysed in 139 patients with chronic plaque psoriasis and in 155 unrelated healthy controls from Thailand. There ware no significant differences in the allele frequencies of any of the four SNPs between patients with psoriasis and controls. However, the frequency of the -2763A allele was increased in patients with late-onset psoriasis compared with controls and patients with early-onset psoriasis [OR=2.94, 95%CI 1.16-7.39, corrected P value (Pc)=0.04 and OR=3.26, 95%CI 1.13-9.51, Pc=0.048, respectively]. The AAGC (-3575/-2763/-1082/-592) haplotype frequency was higher in late-onset compared with early onset psoriasis (OR=4.37, 95%CI 1.24-15.97, Pc=0.027). This study suggests that the -2763A allele and the extended AAGC haplotype can be used as agenetic marker for susceptibility to late-onset psoriasis in a Thai population.