The Simplified Psoriasis Index (SPI) is a summary measure of psoriasis with separate components for current severity (weighted for functionally or psychosocially important sites), psychosocial impact and past behaviour. The professionally-assessed (proSPI-s) and patient self-assessed (saSPI-s) severity components of each have been shown to be valid and reliable. Their responsiveness to change and equivalence to the current standard (Psoriasis Area and Severity Index: PASI) were investigated. Responsiveness and minimum clinically important differences (MCIDs) were derived from PASI changes from baseline at weeks four (n=100) and 10 (n=65) in patients commencing therapy for psoriasis. Receiver operating characteristic (ROC) analysis confirmed that both measures detected responsiveness well (area under the curve [AUC]=0.72-0.96). On ROC and PASI-based anchor analysis, MCIDs equated to mean absolute and percentage changes of 5 and 60% (proSPI-s), and 7 and 70% (saSPI-s). Satisfactory response as defined by 75% reduction in PASI equated to 85 and 95% reductions in proSPI-s and saSPI-s respectively. PASI-equivalent cut-off scores for mild (PASI<10) and severe (PASI>20) psoriasis were <9 and >18 for proSPI-s (n=300) and <10 and >20 for saSPI-s (n=200) (AUC=0.86-0.96). These studies further support the validity of SPI for use in routine clinical practice.Journal of Investigative Dermatology accepted article preview online, 29 July 2013. doi:10.1038/jid.2013.318.