Photodynamic diagnosis and therapy of tumours is a new procedure utilizing the combined action of light and a photosensitizing drug. The laser-fibre optic delivery system allows the visualization and treatment of superficial and deeply located neoplasms including malignancies of the skin, head and neck, oesophagus, endobronchial tract, stomach, urinary bladder, female genital tract and other sites. The putative action mechanism depends on evocation of fluorescence of a photosensitizing drug within cancer tissue (important for non-invasive optical diagnosis) and involves photochemical destruction of tumour tissue membranes mediated by singlet molecular oxygen (important for destruction of tumour cells).