Background:Nevus sebaceus of Jadassohn is a complex anomaly involving the epidermis, dermis, apocrine glands and hair follicles, as well as the sebaceous glands, and a variety of benign and malignant tumors and neurologic abnormalities may be associated with it; however, only a few studies on it have been reported in the Korean literature. Objective: The objectives of this study was to document clinical and histopathologic features of nevus sebaceus in Koreans and compare them to those in the previously reported studies. Method: Clinical records and histopathology of 53 cases of nevus sebaceus diagnosed at Ewha Womans University Tongdaemun Hospital during a period of 18 years were reviewed and analyzed. Results: The average incidence rate of nevus sebaceus among new patients was 0.03%. The male-to-female ratio was 1.04 and the average age at visit, 16.5 years. The lesions had been present since birth or had developed before age 10 in 85% of the cases. A11 lesions were situated on the head, and the most common site was the scalp. Linear 1esions (20.8%) were mostly facial lesions. Histopathologically, the incidence of sebaceous and apocrine glandular hyperplasia was most notable in the 13-18 and over-18 age groups. The sebaceous glands showed small punched out defects or "holes" in 24.5%; ectopic apocrine glands were found in 24.5 %. Mature hair follicles were absent or greatly reduced in number in 75.5%. Acanthosis nigricanslike pattern of acanthosis and mild papillomatosis were the dominant changes in the epidermis. Hyperplasias and neoplasms were associated in 15.1%, which included 5 cases of primitive follicular induction and 1 trichoblastoma. Conclusion: In this series, no examples of malignant neoplasms were identified; however, an early total excision before age 13 is recommended since a variable "deficit malformation" of the follicles with hyperplasia of the epidermis and sebaceous glands becomes more prominent after that age. (Ann Dermatol 13(1) 32~38, 2001).