We have previously reported that the SAPASI, designed for epidemiologic studies, predicts the Psoriasis Area Severity Index (PASI). Our new instrument consists of a body silhouette for patients to shade in affected areas and of three modified visual analog scales for lesional severity erythema, induration and scale. Trained personnel obtained a PASI score on the same day a SAPASI was obtained (80 subjects, 153 observations). To assess criterion validity, regression on the first paired observation found that the SAPASI predicts the PASI (p = 0.0001). Strong correlations (p = 0.0001) were found between SAPASI and PASI for the following variables: body surface area assessments, erythema, induration and scale. To assess the stability and reliability, test-retest within 2 days found strong correlations between first and second PASI scores and first and second SAPASI scores (n = 19, p = 0.0001). To assess reprodicibility of scoring SAPASI body surface area measurements, 5 raters blindly evaluated 40 randomly chosen body silhouettes. The intraclass correlation coefficient (R) for the overall body score was 0.953, whereas individual body area R ranged from 0.84 to 0.96. Our studies of the criterion validity, content validity, reliability, and responsiveness of the SAPASI suggest that instrument is valid for predicting psoriasis disease severity in a population.