Background: The pathogenic role of Demodex is still a matter of debate. We accidentally found large numbers of Demodex from sebacous hyperplasia(SH), Objective : The aim of this study is to compare the number of Demodex in the with control group and to observe the histopathological finding of SH. Methods : Extraction was performed in 11 patients with SH, and in 40 controls. Excision biopsy was performed in 8 patients with SH. Results : The prevalence and density of Demodex was significantly higher in patients with SH (prevalence = 100%; mean density = 28.9/lesion) than in the controls(prevalence = 85%; mean density=2.1/follicle). Histopathologically, a dense homogenous eosinophilic material surrounding the mites(abnormal keratinization of ductal epithelium), sloughing of keratin, follicular plugging, obstruction of the hair follicle and sebaceous duct, cellular destruction of the sebaceous gland(degeneration), squamous metaplasia of sebaceous gland cells, and follicular and perifollicular inflammatory infiltration were observed. Demodex organisms were found in the hair follicle, sebaceous duct and sebaceous gland. Conclusion : The present study demonstrates a high density of Demodex in the lesion of SH and this finding suggests possible relationship of Demodex in the development of the skin lesion of SH. (Kor J Dermatol 1996;34(1): 52-57)