BACKGROUND: Systems for determining psoriasis severity in clinical trials have not been sufficiently validated against patientsʼ perceived quality of life. OBJECTIVE: To validate three systems of physician-determined psoriasis severity (the Lattice System Physicianʼs Global Assessment [LS-PGA], Psoriasis Area and Severity Index [PASI] and static Physicianʼs Global Assessment [sPGA]). METHODS: Data were from a 24-week randomized, double-blind, placebo-controlled, multicenter trial of therapy with oral calcineurin inhibitors in 445 patients. Construct validity was measured by correlations of the three severity scores with patientsʼ self-reported quality of life (QoL) from the Dermatology Life Quality Index (DLQI) and a DLQI item about psoriasis symptoms. RESULTS: All severity systems were moderately and positively correlated with QoL, indicating construct validity. QoL was most consistently related to physiciansʼ assessments of body surface area involved with psoriasis (iBSA) followed by, in the order of consistency, plaque elevation, erythema and scale. CONCLUSIONS: The LS-PGA weights iBSA and aspects of plaque morphology in concert with their relative effects on QoL. The LS-PGA, sPGA and PASI are validated by their relationship to QoL in a clinical trial.