Photodynamic therapy (PDT) involves the use of photosensitizing agents for treatment of malignant disease. These photosensitizing agents are infused intravenously and are selectively retained within tumor cells. The agents remain inactive until exposed to light of the proper wavelength. When activated by light, these compounds generate toxic oxygen radicals that result in tumor necrosis. Although several institutions worked with PDT in the 1980s, its use in the United States remained limited to research. More recently, PDT using the first FDA approved photosensitizing agent, porfimer sodium (Photofrin), has become available for routine clinical use in the United States.