The causes of melanoma with leg weakness include brain metastasis of melanoma, bone metastasis in the legs, electrolyte disorders, severe anemia or malnutrition. 1. Melanoma metastases to the brain affect the motor center of the legs, resulting in weakness of the legs. 2. When melanoma metastasizes to the leg tissues through the bloodstream pathway, the patients usually have symptoms such as leg weakness and local pain. 3. In advanced stage, some patients may suffer from complications of electrolyte disorders such as hypokalemia and hyponatremia, and sometimes they may also show leg weakness. 4. When melanoma develops into advanced stage, some patients will have complications such as severe anemia or malnutrition, which will also manifest leg weakness.