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Background: Traditionally, treatment with the excimer laser requires determining the minimal erythema dose on healthy skin or using plaque-based induration; however, these protocols often lead to underdosing of psoriatic plaques and reduced treatment efficacy. Objective: To prospectively evaluate the effect of the excimer laser on plaque psoriasis using an optimal therapeutic dose (OTD) protocol. Methods: Subjects with stable plaque psoriasis were tested with the Multi-Microdose (MMD) tip on the XTRAC excimer laser to determine a minimum blistering dose (MBD). Treatment was then initiated at 20% less than the MBD. A single psoriatic lesion was treated once weekly for up to 11 sessions. The change from baseline of the target lesion's modified psoriasis area severity index (mPASI), quality of life and safety were evaluated. Results: Thirteen subjects with a mean age of 48.9±14.9 years and Fitzpatrick skin types I-IV participated in the study. Target plaque mPASI significantly decreased at all time points relative to baseline with significant improvement by the second treatment. Patients reached mPASI-75 within 5±2 sessions. By the end of the study 92% of patients achieved mPASI-75. On average, patients maintained an mPASI score ≥50% for 60 days. Treatment was well tolerated with no erosions or hyperpigmentation. Erythema was the most common adverse event. Conclusion: The OTDTM protocol with the MMD® tip allows determining the optimal dose locally on the psoriatic plaque itself. Consequently, ineffectual dosing levels and treatments are minimized. The OTD protocol reduces treatment frequency from 2-3 times per week to once weekly. J Drugs Dermatol. 2020;19(4):349-354. doi:10.36849/JDD.2020.4891.