Psoriasis risk increased by smoking, not caffeine

By 2005, 896 cases had developed. Participants were asked about their daily intake of foods and beverages during the previous year at four-year intervals. 

Women with higher consumption of caffeine were more likelyto be current smokers and had a higher quantity of alcohol intake. The risk of psoriasis was moderately elevated with both increasing coffee consumption and higher caffeine intake in the age-adjusted mode, but the trend became non-significant once it was adjusted for smoking.

Likewise, decaffeinated coffee was not significantly associated with risk of psoriasis. 

Arch Dermatol 2012; 148:395-97