Recent genome-wide association studies (GWAS) of many complex diseases have successfully identified novel susceptibility loci, with many of them being associated with more than one condition. Taking into consideration that different autoimmune diseases may share some common pathogenetic pathways, we hypothesized that STAT4, a susceptibility gene found to be associated with increased risk for systemic lupus erythematosus, rheumatoid arthritis, type 1 diabetes, Sjögren's syndrome, Wegener's granulomatosis, Crohn's disease, and ulcerative colitis may also have a role in psoriasis. Psoriasis is an autoimmune, chronic inflammatory skin disease. Here we performed a case-control study in the population of island of Crete and demonstrated for the first time the association of a STAT4 single nucleotide polymorphism (SNP) with susceptibility to psoriasis, thus suggesting a putative key role of STAT4 in multiple autoimmune diseases. We found that mutated allele T of the STAT4 rs7574865 SNP, which previously was implicated in the predisposition to many autoimmune diseases, were more common in individuals with psoriasis than in controls (p = 0.045, odds ratio = 1.42, 95% confidence interval 1.01-2.00), thus concluding that the polymorphism examined is associated with the development of psoriasis in our population.