Photodynamic therapy (PDT) uses drugs which, while pharmacologically inactive, can be activated in vivo by visible or near infrared light to produce a local toxic reaction (photosensitizers). Photofrin II (PII) is a mixture of oligomeric porphyrins which accumulate and are retained over several days in all malignant tissue at levels generally higher than surrounding epithelial tissues. Following activation by red light, generally obtained from a laser and delivered by simple quartz fiber optics, PII produces a photochemical generation of cytotoxic singlet oxygen that destroys the tissue in which it resides. Several thousand cancer patients with both early and advanced tumors have been treated to date with encouraging results. Phase III comparative, controlled clinical trials are underway for photodynamic treatment of tumors of the bladder, esophagus, and bronchus.