Photodynamic therapy (PDT) is a promising technique for producing localized destruction of small cancers in the gastrointestinal tract. Following prior intravenous administration of a photosensitizing drug, the tumour area is exposed to low power red light from a laser. There is a small degree of selectivity for tumour areas, but even if there is damage to normal tissues, this heals by regeneration. Thus the tumour and a surrounding cuff of normal tissue can be treated without the need for surgery and with safe healing of all treated areas without risk of perforation. There are 2 main problems. PDT can only be applied to small tumours (up to 1-2 cm thick) partly because the red light used only penetrates a few mm into tissue, but also because there is a risk of delayed haemorrhage following partial necrosis of large lesions. However, it may be complementary to other techniques. If the main bulk of a cancer is removed by surgery, PDT may be able to destroy any small areas of remaining cancer that are not accessible for resection. The other problem is that the most frequently used photosensitizer, haematoporphyrin derivative (HpD), leaves the patient sensitive to sunlight for several weeks after treatment. This will be much less of a problem with the newer photosensitizers such as aluminium sulphonated phthalocyanines and amino laevulinic acid, both of which should be ready for preliminary clinical trials within a few months. PDT has been applied on its own for the palliation of dysphagia from advanced oesophageal cancers, but the results as far are purely anecdotal and it seems unlikely that this will prove a useful approach unless PDT is combined with other therapies. The potential for PDT in treating gastrointestinal tumours is great, but a careful understanding of the biology involved is essential before this can be realized.