How often does a cancer develop in a mole?

The majority of moles are harmless but in rare cases the moles can turn into a cancer. Because of the concern about moles and skin cancer, these skin lesions must be monitored for changes in skin color and size. Skin cancerous moles can turn into a malignant melanoma. However it should be understood that not all melanomas develop from preexisting moles, but many develop from sites in an around a preexisting mole.