Accumulating evidence indicates that body weight, alcohol and smoking are associated with psoriasis. However, these factors have scarcely been investigated in relation to onset and disease activity at onset of psoriasis. A population-based case-control study was performed including 373 cases with onset of first-time plaque psoriasis within 12 months and matched healthy controls. Psoriasis activity was measured using the Psoriasis Area and Severity Index (PASI). Analyses were performed using conditional logistic regression. In multivariable analyses for each unit increment in body mass index, there was statistically significant 9% increased risk for psoriasis onset and 7% higher risk for increased PASI. Obesity (body mass index > or =30) compared with normal body weight was associated with a two-fold increased risk for psoriasis onset. Smoking was associated with a 70% increased risk for onset, but was not related to PASI. A positive association with alcohol drinking was observed among men, but not among women. No associations were observed for weight gain and use of smokeless tobacco. Our results indicate that excessive body weight and smoking are risk factors for onset of psoriasis and that higher body mass index increases the PASI of plaque psoriasis at onset.