Background: Optimal treatment of primary Merkel cell carcinoma (MCC) is unknown. High local recurrence rates after excision alone compel some physicians to advocate postoperative radiation therapy to improve local control. Objective: We wondered whether marginal recurrence and survival rates differed between patients with primary MCC treated with Mohs surgery alone and those treated with Mohs surgery and adjuvant postoperative radiation. Methods: A collaborative retrospective study was performed; the study group consisted of 45 patients with stage I MCC who were histologically and clinically free of disease after Mohs excision. Twenty patients subsequently received elective postoperative radiation to the primary site, and 25 patients had no adjuvant radiation therapy. Results: One marginal recurrence (4%) and 3 in-transit metastases were observed in the Mohs surgery alone group, whereas none were observed in the Mohs surgery and radiation group. The proportion of patients with these events was not significantly different between treatment groups. Overall survival, relapse-free survival, and disease-free survival were not significantly different between treatment groups. Conclusion: Adjuvant radiation appears unessential to secure local control of primary MCC lesions completely excised with Mohs micrographic surgery. Adjuvant radiation is recommended for patients unable to have complete excision or if complete histologic margin control is unavailable and should be considered for patients with large or recurrent tumors. (J Am Acad Dermatol 2002;47:885-92.)