Photodynamic therapy (PDT) is a cancer treatment modality, based on the synergistic photochemical reaction between tumor cells containing accumulated photosensitizer and laser light. The mechanisms of killing of malignant cells were studied by several researchers (1 –3 ). Photosensitizers accumulate more in malignant cells than in normal cells, and this concentration difference contributes to the selective killing of the malignant cells. In general, the transmissivity of light in tissue seems to be low and a high light energy delivered for a long time causes the thermal change in tissue. Therefore, provided the malignant tumor is treated with a low light energy to avoid thermal effects, the use of a highly transmissible laser light should be favorable. Pulsed laser light is considered to be the best regimen in PDT (4 ), and an excimer laser-pumped dye laser (abbreviated as an excimer dye laser) seems to give better light transmission than a continuous-wave argon laser pumped dye laser (4 ). The effect of PDT with the photosensitizer Photofrin II and the excimer dye laser was investigated in Japan with mouse tumors (5 ) and human tumors clinically. PDT by means of Photofrin II and the excimer dye laser was offi cially approved by the Japanese government at October in 1994 for the treatment of superfi cial diseases such as lung cancer, esophagus cancer, stomach cancer, cervical cancer and its dysplasia. Cure rate of more than 80% was obtained in the offi cial investigational clinical study (6 –9 ). This success encouraged us to try the application of PDT to nonsuperfi cial cancers and we applied the interstitial PDT to treat these cancers.