Background: Margin evaluation of melanoma in situ (MIS) is difficult because of its ill-defined clinical borders. Wood's light examination is commonly used to help delineate MIS margin before excision. Objective: To prospectively study the accuracy of preoperative Wood's light examination for margin assessment of MIS. Materials and methods: The authors evaluated 60 patients before excision of MIS under white light and Wood's light. Staged excision was performed using the square procedure technique. After achieving clear margins, they compared final wound size with expected wound size if surgical margins had been based on Wood's light examination. Results: Seven patients (11.7%) had Wood's light enhancement beyond the visible margin of the biopsy site. In all 7, increased wounding would have occurred if the surgical margins had been based on Wood's light examination. In 1 of the 7, use of the Wood's light examination would have reduced the surgical stages needed by 1 stage but would have increased the wound size by 83.3%. Conclusion: Wood's light examination has limited utility if complete excisional biopsy of MIS is performed before treatment. In this study, surgical margin based on the Wood's light examination would have resulted in an increased average wound size and would not have reduced the number of stages needed when performing the square procedure.