The incidence of metastasis in cutaneous melanoma is proportional to maximal tumor thickness. The relationship is linear for extremity melanoma but not for tumors from all body sites, probably due to intrinsic differences in tumors from different sites. The level of invasion of the tumor is an indirect measure of tumor thickness and is not as accurate in predicting metastases because of the marked variation in thickness within each level. The implications of these observations for the treatment of melanoma are discussed.