Background: Longitudinal melanonychia (LM) can be challenging as it may be caused by a wide variety of benign and malignant conditions. However, there are scarce data on LM confirmed by skin biopsy examination in Korean patients. Objective: We sought to evaluate the clinical features and histopathologic diagnosis of LM in Korean patients. Methods: We reviewed the medical records and clinical photographs of patients presenting with LM confirmed by skin biopsy examination between June 2007 and June 2017. Results: Among a total of 75 patients with LM confirmed by skin biopsy examination at our hospital over a period of 10 years, 11 patients (14.7%) had malignant lesions. Common features of malignant lesions were involvement of a single nail, color heterogeneity, Hutchinson’s sign, and nail plate dystrophy or ulceration. Common features of benign lesions were a negative Hutchinson’s sign and absence of nail plate dystrophy and ulceration. Conclusion: This study analyzed the clinical features and histopathologic diagnosis of LM in Korean patients. Our data may contribute to determining the management approach for patients with LM. (Korean J Dermatol 2018;56(3):172∼180)