The self-administered psoriasis area and severity index (SAPASI) is a structured instrument for measuring the severity of psoriasis. This study examines the validity, reliability, and responsiveness of the SAPASI. Trained personnel performed a psoriasis area and severity index (PASI) assessment on the same day a SAPASI was obtained from 80 subjects. The validity of the SAPASI was demonstrated using the PASI as the standard (r2 = 0.59, p = 0.0001). Significant correlations were found between SAPASI and PASI for body surface area (r = 0.62-0.75), erythema (r = 0.39), induration (r = 0.24) and scale (r = 0.38). Test-retest reliability was assessed in 19 subjects with repeated evaluations within 2 d. Correlations between the first and second SAPASI scores (r = 0.82) were highly significant (p = 0.0001). Inter-rater reliability of SAPASI body surface area measurements among five raters was very high (intraclass correlation coefficient R = 0.953). The SAPASI was responsive to changes in severity over time as demonstrated by correlation with changes in PASI scores (r = 0.63, p = 0.0002). We conclude that this structured patient self-report instrument facilitates quantitative assessment of psoriasis.