Dyes have roles in the aetiology, diagnosis, and treatment of cancer.

Synthetic dyes particularly coal tar dyes can be carcinogenic.

Dyes are used to identify cancer, and are especially useful in intra-operative imaging.

Dyes inspired the birth of modern chemotherapy, and can be used as chemo-ablative agents.

Theranostics is a new field where the same agent (such as a dye) can be used for both diagnosis and therapy.