Cases of malignant melanoma with multiple skin metastasis are not always uniformly progressive. In some cases the condition appears to remain stationary for years; old lesions die off and new ones appear. When a lesion like this involutes, it dries up and a crust falls off, leaving a pink mark. Microscopical section may show a few residual malignant melanocytes in the dermis, together with increased vascularity, well-marked round cell infiltration and many melanophages, the result of previousactivity. Regression is more commonly seen in part ofa malignant melanoma, as for instance when part of the pigmented patch blanches. Such tumours have a better than average prognosis. Regression or involution of a malignant melanoma can be explained in terms of cellular immunity.