We developed a structured Psoriasis Area Severity Index (PASI)-like instrument, the Self-administered PASI (SAPASI), that allows subjects to assess accurately the severity of their psoriasis. The major limitation of our previous SAPASI validity studies is that all were performed in a single academic center, raising questions about the generalizability of the instrument. We administered the SAPASI to 182 subjects in a 12-week, multicenter, double-blind clinical trial of topical tazarotene for psoriasis. On the same day, investigators blind to the SAPASI rating determined the degree of erythema, induration, scale, body surface area (BSA) affected, and overall lesion severity of the subjects' psoriasis. Using these data, we calculated an investigator PASI-Equivalent. Correlation analysis shows that for both initial and final assessments of psoriasis severity, the SAPASI score reflects the PASI-Equivalent score in a significant way (p = .0001), although the correlation is a modest one (r = 0.3 to 0.5). Significant (p = .0001), modest correlations were found between the subjects' reported BSAs and the investigators' reported BSAs. To assess responsiveness, the proportional changes of the SAPASI and PASI-Equivalent were found to be modestly significantly correlated (r = 0.2, p = .04). The results of this study support the general validity of the SAPASI and demonstrate that the SAPASI can detect changes in disease severity in a clinical trial. Significant correlations were also observed between SAPASI components and their investigator-reported counterparts in this multicenter trial. To the best of our knowledge, the current study represents the first multicenter validity study performed on a psoriasis severity instrument, and clearly demonstrates the value of this instrument in assessing the psoriasis severity in a population.