Photochemotherapy of cancer is a new modality of cancer treatment now being evaluated in clinical trials worldwide. It is based on injection of photosensitizing and tumour-localizing dyes followed by exposure of the tumour region to intense light, usually from a laser. A large number of different types of tumours respond to photochemotherapy. However, for photochemotherapy to be efficient, it is absolutely essential that the entire tumour can be reached by light. Thus photochemotherapy is rarely efficient for tumours thicker than 5-7 mm, unless the light is applied interstitially through fibres. Fibre optics can also be used for endoscopic applications. It has been reported that extracorporeal photochemotherapy of T-cells, using 8-methoxypsoralene as sensitizer, has given good clinical results in patients suffering from the erythrodermic form of cutaneous T-cell lymphoma. Extracorporeal photochemotherapy for elimination of residual tumour cells from autologous bone marrow grafts is also being evaluated.