The current grouping of patients with malignant melanoma into thin, intermediate, and thick melanomas provides a convenient but arbitrary classification which, although providing "average" survival values for each group, offers crude prognostication for the individual patient. A review of 371 patients with clinical Stage I malignant melanoma, treated during the period 1970 to 1985, was conducted. The estimated 5-year survival rate for female patients with melanomas 1.0 mm thick was 94%; for each 1-mm increment in thickness the survival rate declined by about 3%, up to the 6 mm mark, the survival rate declining thereafter by about 8% for each additional millimeter in the range of 7 to 15 mm of thickness. The estimated 5-year survival rate for male patients with melanomas 1.0 mm thick was 80%; for each 1-mm increment the survival rate declined by about 970, up to the 10 mm mark. The proposed method of estimating the expected survival according to the patient's sex and the thickness of the primary lesion hopefully provides a more accurate and convenient method of prognostication for the clinician dealing with specific patients with intermediate or thick melanomas. Cancer 64:1432-1436, 1989. HE MICROSTAGING METHODS developed by Clark et T al. I and Breslow' have led to major progress in our understanding and prognostication of malignant mela-noma. Both methods are associated with survival time and are used simultaneously in the evaluation of a primary mel-anoma. However, the two methods do not correspond exactly; occasionally Clark's level 111 or IV melanomas are less than 1 .O mm in thickne~s.~ Various studies indicate that thickness provides a more accurate prognostication which cannot be improved further with the additional information provided by Clark's m e t h ~ d. ~-~ In a previous review of patients from Roswell Park Memorial Institute (Buffalo, NY) with clinical Stage I mel-anoma, it was found that thickness was the most important prognostic indicator for survival time, followed in significance by the sex of the patient. Other factors such as location of the lesion, type of surgery, age, lymphocytic infiltration, number of mitoses, vascular invasion, and presence or absence of ulceration were not significant as prognostic indicators when a stepwise proportional haz