In this article, we first present our clinical data on PDT for the treatment of gastric cancer and make a comparison between a continuous wave laser and a pulsed laser. The reasons for PDT failure in certain cases are also discussed. In the fifteen years from 1981 to 1995, we have treated a total of 76 gastric cancer lesions (73 cases), which was consist of 69 early gastric cancer lesions (66 cases) and seven advanced gastric cancer lesions (seven cases) by PDT. From 1981 to 1990, we used an argon dye laser (ADL, Models 171-08 and 375-03, Spectra-Physics, Mountain View, Calif., US) as an excitation light source for PDT with HpD (Photofrin I), DHE (Photofrin II) or PHE (freeze-dried Photofrin II). From analysis of the results in terms of the depth of cancer invasion in these 44 lesions (41 cases), the rate of cure for mucosal carcinomas was 57% (13/23), that of submucosal carcinomas was 53% (10/19), and that of carcinomas invading more than the muscularis propria was 0% (0/2). These data can be interpreted to indicate that the ADL laser beam could not penetrate and supply sufficient energy to activate HpD not only in the submoucosal layer but also in the mucosal layer. In 1990, therefore, we investigated an excimer dye laser (EDL, Hamamatsu Photonics, Hamamatsu, Japan), because its pulsed beam with extremely high peak power was expected to be more efficient at exciting HpD than continuous wave lasers such as ADL and high frequency pulsed lasers such as cooper vapor dye laser (Cu VDL). From 1990 to 1995, twenty-seven early gastric cancer lesions (27 cases) and five advanced gastric cancer lesions (five cases) were treated by PDT with EDL and PHE. Of these 32 lesions, the rate of cure for mucosal carcinomas was 100% (15/15), that of submucosal carcinomas was 75% (9/12), and that of carcinomas invading more than the muscularis propria was 20% (1/5). For the purpose of determining how much energy was required for a complete cure in early gastric cancer, and to compare the efficacy of ADL and EDL, the relation between the response (cure or no cure) and irradiated energy intensity (dose: J/cm 2) was evaluated by the depth of cancer invasion and kind of laser used in PDT. A smaller EDL dose was more effective than ADL in terms of photodynamic action.