What happens if skin cancer is left untreated?

Metastasis: Malignant melanoma is largely curative if caught and excised very early. However, if left untreated, it will metastasize (spread to distant areas), and unfortunately, often fatal. Aggressive forms of squamous cell carcinomas at sites such as the ears and scalp can also metastasize or spread via the underlying nerves if left untreated.

Local destruction: If the skin cancer in question is a basal cell carcinoma, although typically doesn't metastasize (spread to distant areas), it can cause significant local tissue destruction by direct extension. For example, if it occurs at an anatomically important area such as the skin around the eye, by not treating and allowing it to track down the orbit, it will ultimately result in loss of the eye.

It will spread: Different cancers spread differently. All cancers will grow locally and cause damage to nearby structures. Cancers can spread to lymph nodes which drain a particular body area as well (i.e. Squamous cell, and melanoma). Some cancers (particularly melanoma) will jump to distant sites early (brain, lung, liver, etc). This is distant metastasis, and imparts poor prognosis.

Progress: The skin cancer will progress and become larger. In the case of melanoma it may travel through the body and create metastatic disease.