Background: Acquired bilateral nevus of Ota-like macules (Hori's nevus) is a dermal pigmented lesion commonly seen in middle-aged women of Asian descent. The Q-switched ruby laser (QSRL) has been used successfully to treat a variety of benign pigmented lesions. Multiple, sequential treatments are typically required for complete clearance of the dermal pigmented dermatoses. Objective: The purpose of this study was to determine the efficacy of QSRL in the treatment of Hori's nevus and the beneficial effect of epidermal ablation using the scanned carbon dioxide (CO₂) laser before QSRL. Methods: A total of 13 women from Thailand with Hori's nevus were randomly treated with the scanned CO₂ laser followed by QSRL on one side of their face, and QSRL alone on the other side. The same fluence of QSRL was used on both sides in individual patients. The treatment response was objectively evaluated by measuring the melanin index using a Mexameter (Courage & Khazaka Electronic GmbH, Köln, Germany), and subjectively assessed by the patients before treatment and 3 and 16 months after treatment. Adverse sequelae of the treatment and the patients' tolerance were also evaluated at the same follow-up visit. Results: The 3- and 16-month posttreatment melanin index was significantly decreased compared with that of pretreatment on both treated sites and this corresponded to the patients' subjective evaluations. The response rate, defined as “the percentage of reduction in melanin index,” was significantly higher on the sides treated with scanned CO₂ laser followed by QSRL, compared with the sides irradiated with QSRL alone at both follow-up visits. At the 3-month follow-up, the most common adverse effect was hypopigmentation, found in 15% (2 of 13) of the patients on the sites treated with QSRL alone, and on the sites treated with scanned CO₂ laser followed by QSRL (8%, 1/13). Erythema was observed in 15% (2/13) of the patients only on the sites that received combination treatment. However, no adverse sequelae were observed at the 16-month posttreatment follow-up. Conclusion: Epidermal ablation with scanned CO₂ laser before the use of the pigment-specific laser may be an effective technique for increasing therapeutic efficacy in the treatment of dermal pigmented dermatoses. (J Am Acad Dermatol 2003;48:584-91.)