Background : Nucleolar organizer regions(NORs) are loops of DNA that transcribe ribosomal RNA; they can be easily identified in paraffin section using silver method. Recently, the application of NORs to the study of various types of benign and malignant proliferation has suggested that the number of NORs per nucleus can provide an indication of degree of cellur maligriancy. Many authors indicated a significant overlapping of NORs counts between benign and malignant proliferation. Objective : The purpose of this study is to discriminate benign m laocytic nevi from malignant melanoma effectively, thus we use simple enumeration and computerized image analysis of AgNORs in melanocytic skin lesions. Method : NORs were investigated on silver stained histologicec ion of 18 malignant melanoma (MM), 19 acquired nevi(AN), 21 congenital nevi(CN). For each case, 100 nucleus sample were cuantified using image analysis system(AIC Inc., Roswell, GA). There is no consensus at present as to the best criterion for quantifying AgNORs proteins in melanocytic skin lesion. We prefer to quantify a large number of parameters. The mean value or standard deviation(SD), each calculated for a batch of 100 cells, are average again(mean+SD), over the 58 cases for six morphological criteria mean number of AgNORs per nucleu, mean ratio of AgNORs area per ruc eus area, mean area of largest AgNORs, mean ratio of largest AgNORs area per nucleus area, mean nucleus area per a AgNOR, and coefficient of variation of nucleus area. Results : 1. All of the six parameters show significant difference between benign melanocytic nevi and malignant melanoma. But., there is no significant difference between acquired nevi and congenital nevi. 2. In stepwise discriminate analysis, we discriminate benign melanoctic nevi from malignant melanoma effectively using combined two parameters(number of AgNORs and mean ratio of AgNORs area per nucleus area). But there is some overlapping between malignant melanoma and ber ign melanocytic nevi using each six parameters. 3. In metastatic and non-metastatic malignant melanoma, six param ters have not shown significant difference. Conclusion : Study of AgNORs in the image analysis system is a very useful tool to differentiate malignant melanoma from benign melanocytic nevi. (Kor J Dermatol) 1993;31(6):866-876)