In 2012 patients with primary malignant melanoma of the skin the primary lesion was reviewed microscopically and the depth of invasion was staged and the tumor thickness was measured. It was possible to improve the prognostic value of Clark's method by dividing Clark's level IV (reticular dermis) into 2 subgroups, level IV A and level IV B, corresponding to the upper half and the lower half of the reticular dermis, respectively. Level IV A lesions were correlated with a significantly better prognosis than level IV B lesions (p less than 0.0025). By combining depth of invasion and tumors thickness it was found that for each level of invasion (except for level II) the prognosis deteriorated significantly with increasing tumor thickness. Level II indicated an excellent prognosis regardless of tumor thickness. Conversely, it was found that for each subgroup of thickness (except for 0.76 - 1.50 mm) the prognosis was not influenced by level of invasion. In melanomas measuring 0.76 - 1.50 mm in thickness the survival rate was significantly deteriorated, the deeper the invasion (p = 0.0003). The prognostic value of measuring tumor thickness was superior to that of staging depth of invasion. In melanomas 0.76 - 1.50 mm thick, however, additional classification of depth of invasion improved the prognostic value significantly.