Background: Little is known about the long-term prognostic factors which affect clinical outcomes after the diagnosis of psoriasis. Objective: To identify the prognostic factors which predict long-term outcomes after diagnosis. Methods: Patients’ clinical characteristics at diagnosis were associated with long-term prognosis using the logistic regression method for 169 psoriasis patients (median follow-up time 11.6 years). Results: The factors age ≧40 [odds ratio 0.27, 95% confidence interval (CI) 0.10–0.72, p = 0.0091], male (odds ratio 2.67, 95% CI 1.02–7.00,p = 0.0459) and BMI ≧25 (odds ratio 2.74, 95% CI 1.01–7.42, p = 0.0480) had significant effects on long-term prognosis after diagnosis. A simple prognostic index based on these factors was proposed. Conclusions: Three major prognostic factors were identified. The proposed index may be useful in the design of future clinical trials for psoriasis and in the selection of appropriate therapeutic approaches for individual patients.