Photodynamic treatment (PDT) of malignant tumours rests on the intravenous administration of a photosensitizer, which is predominantly in tumour tissue, and subsequent activation of this substance with light of a specific wavelength. The method was used in a 76-year-old man who had undergone a 2/3 gastric resection for ulcer in 1948 and was now found to have a carcinoma of the stump at the anastomosis. He refused surgical resection, but accepted PDT. 48 hours after intravenous administration of the photosensitizer dihematoporphyrin ether-ester (2 mg/kg) the 1.5 x 1.0 cm tumour was irradiated with an argon ion laser pumped colour laser system at a wavelength of 630 nm and energy density of 150 J/cm3. The irradiation was performed under endoscopic control via a specially developed applicator which had first been introduced through the biopsy channel of the endoscope. Four weeks after the first treatment biopsies from a 5 mm ulceration in the area of the tumour revealed malignant cell groups, so that a second treatment was undertaken. Two, three and six months later the ulcer was shown to have healed and biopsies were free of tumour. But nine months later tumour cells were again demonstrated and a third PDT was performed. Although biopsies were again free of tumour at the next control, it can not yet be assumed that the tumour has been cured.