Overweight is a proposed risk factor for psoriasis. How-ever, evidence from prospective studies is limited. The aim of this study was to investigate the association be-tween overweight, weight gain and risk of psoriasis, and potential synergism with smoking, within a population-based cohort including 8,752 individuals followed from 1994 up to 2008. There was a 32% increased odds of psoriasis from a body mass index (BMI) of 27 kg/m2, in multi-variable logistic regression analysis, further increasing to 43% at BMI 28 kg/m2, and to 71% at BMI ≥ 30 kg/m2 in non-smokers. There was a dose-response association between weight gain from age 25 years, with up to 90% higher odds of psoriasis from middle age, independent of weight category. There was no indication of a synergism between overweight and smoking, and no interaction with sex. Overweight and weight gain represent modifiable risk factors that may be targets for primary prevention of psoriasis.