The metastasizing potential of a malignant tumor is derived primarily from its biological characteristics. The second most important factor in metastatic spreading lies in the host tissue/organ and host metabolism. The aggressiveness of a malignant skin melanoma depends clinically upon the width and, especially, the depth of the preceding lesions(s). Histological and electron microscopic studies reveal that the increase of malignant aggressiveness is directly related to the arrangement of the initial lymphatic vessels in the different layers of the skin. Sudden increase in the density of lymphatic vessels permits a corresponding increment of malignant seeding via the lymphatic system.