Abstract: Statistical analysis was made on the family history of 3149 patients with psoriasis patients. It is estimated that the heritability and standard error of psoriasis was between 38±1.2-48±1.2%. It is thought that the psoriasis is a heriditary polygenic disease and that environmental factors may not be overlooked. An analogy of Poisson distribution model was performed by probability density function for 408 psoriasis families in some region in Changchun, Jilin province. We found that there was significant clustering of cases in the psoriasis families. It is suggested that psoriasis is related with genetic predisposition (susceptibility or liability) and other environmental factors.