The cause of psoriasis has only recently been subject to structured epidemiologic research approach. This is reflected in the scarcity of quantitative knowledge about etiologic factors for psoriasis other than genetics. A schematic model suggests that susceptibility to develop psoriatic lesions includes both genetic and extragenetic predisposing factors. Expression in terms of visible lesions is modulated by precipitating or trigger factors. Unlike many chronic diseases, psoriasis spontaneously waxes and wanes. When assessing factors that trigger the onset of the disease, one must also consider factors that lead to recurrence or exacerbation. Cohort and case-control studies are the main methodologic options for risk assessment. Combination of these techniques and the merging of an epidemiologic approach with basic biologic research are suggested routes to valuable results in this area.