Photodynamic therapy (PDT) is a form of cancer treatment based on the selective accumulation of a photosensitizer (by exogenous or endogenous means) in neoplastic tissue. Subsequent activation of the photosensitizer by a specific wavelength of light results in tumor cell death. Activation of a photosensitizer to the appropriate energy state results in the production of singlet oxygen, a powerful oxidizing agent. PDT can kill cells by three mechanisms: direct cell death by photooxidation, apoptosis, or as a consequence of vascular shutdown. The toxicity of PDT is site specific and dependent on the organ being irradiated and the selectivity of the photosensitizer for target tissue over normal tissue. However, there are also reactions related to the sensitizer per se that are independent of those related to the treatment site. Such reactions include cutaneous photosensitization, nausea, vomiting, hypotension, and altered liver 'function' tests. Excitation of photosensitizer by an incident photon produces reemission of a fluorescent photon, which can be used to detect a tumor that is not ordinarily evident. The major limiting factor in using PDT is the depth of tumor kill. The majority of clinical experience involving PDT of the gastrointestinal tract involves patients who are considered to be poor operative risks, and reported follow-ups after treatment are not only limited but also variable.