Objective: Our purpose was to demonstrate the efficacy of the 308-nm excimer laser for treatment of psoriasis. Methods: This study was a multicenter open trial from 5 dermatology practices (one university-based and 4 private practices). Up to 30 patients per center with stable mild to moderate plaque-type psoriasis constituted the study population. Patients received 308-nm ultraviolet B doses to affected areas. The initial dose was based on multiples of a predetermined minimal erythema dose. Subsequent doses were based on the response to treatment. Treatments were scheduled twice weekly for a total of 10 treatments. The main outcome measure was 75% clearing of the target plaque. Time to clearing was analyzed by Kaplan-Meier methods, accounting for truncated observations. Results: One hundred twenty-four patients were enrolled in the study, and 80 completed the entire protocol. The most common reason for exiting from the study was noncompliance. Of the patients who met the protocol requirements of 10 treatments or clearing, 72% (66/92) achieved at least 75% clearing in an average of 6.2 treatments. Eighty-four percent of patients (95% confidence interval [CI], 79%-87%) reached improvement of 75% or better after 10 or fewer treatments. Fifty percent of patients (95% CI, 35%-61%) reached improvement of 90% or better after 10 or fewer treatments. Common side effects included erythema, blisters, hyperpigmentation, and erosions, but they were well tolerated. Conclusions: Monochromatic 308-nm excimer laser treatment appears to be effective and safe for psoriasis. It requires fewer patient visits than conventional phototherapy, and, unlike those treatments, the laser targets only the affected areas of the skin, sparing the surrounding uninvolved skin. (J Am Acad Dermatol 2002;46:900-6.)