Introduction: Since there is no study in our country about determination the effects of psychogenic factors on psoriasis and because it seems necessary to evaluate the triggering factors of disease in order to lengthen the remission period and decrease the costs and complications, this study was performed to identify the relationship of stress with psoriasis expression and the severity of skin lesions. Materials and Methods: This is a prospective case-control study with simple random sampling which has been performed on 95 subjects above 18 year old in Razi Hospital. The case group was selected from psoriatic patients and the control group from non-psoriatic patients in whom stress does not have any effect on their disease. Two groups were matched for age and sex. Stress was assessed by Holmes and Rahe protocol with some modifications approved by a psychiatrist. Severity of skin lesions in psoriatic patients was also assessed by PASI score. Data Analysis was performed by T test, multiple logistic regression and correlation of Spearman using SPSS (6) software. Results: The mean of severity of stress in psoriatic group was higher than control group (P=0.021). On the other hand, there was a relationship between stress and psoriasis expression; but there was no statistically significant correlation between severity of stress and severity of skin lesions in psoriatic patients (P=0.268). Discussion: Paying attention to the effect of stress on the expression of psoriasis, it is suggested to use psychological methods in addition to routine treatment: So we can reduce emotional stress and shorten the course of this disease.