OBJECTIVES: Research about the limitations of cognitive processes in memory suggest retrospective recall may be less accurate than recall within the past 24 hours.  Daily diaries, however, are often more burdensome for study subjects and are more expensive to administer than weekly retrospective instruments. We sought to evaluate a 24-hour recall version (24-hour PSI) and a 7-day retrospective version (7-day PSI) of the Psoriasis Symptom Inventory (PSI) to test the equivalence of symptom severity assessed with the two versions. METHODS: Using a prospective, observational design, adult patients receiving usual care for their plaque psoriasis (Psoriasis Area and Severity Index [PASI] ≥10) were recruited from clinics and randomized. Subjects completed the 24-hour PSI for 7 days and completed the 7-day PSI on the seventh day.  Comparison of average daily diary scores and 7-day retrospective scores were made with Pearson’s correlation coefficients and simple bivariate comparisons (t-test). Potential exposure effects of daily PSI administration in the days preceding the 7-day retrospective assessment were examined in sensitivity analyses. RESULTS: Among the 139 subjects, mean age and PASI scores were 51.2 and 17.6, respectively. There was high agreement between results from the daily 24-hour PSI and the 7-day PSI assessments. With the exception of Flaking (0.08, p=0.02), no significant mean differences were found in the remaining seven PSI symptoms (Itching, Redness, Scaling, Burning, Stinging, Cracking, and Pain) between the 24-hour PSI (7-day average) and the 7-day PSI. Correlations between the two PSI versions ranged from 0.86 (scaling) to 0.92 (pain). Completion of the daily assessment did not influence responses on the 7-day PSI. CONCLUSIONS: Overall findings show that the two PSI versions yield equivalent results.  Both the 24-hour PSI and 7-day PSI would provide comprehensive capture of symptoms in psoriasis studies and clinical trials.