BACKGROUND AND OBJECTIVE: Under standardized conditions (drug and light dose, timing), the result of the photodynamic therapy (PDT) of carcinomas of the esophagus with tetra(meta-hydroxyphenyl)chlorin (mTHPC) shows large variations between patients. STUDY DESIGN/MATERIALS AND METHODS: Before patients underwent PDT treatment, the mTHPC level was measured in the lesion, the normal surrounding tissue, and the oral cavity, with an apparatus based on fluorescence spectroscopy. RESULTS: The fluctuations in degree of tumor destruction between patients can be explained by individual variations in the mTHPC level in the mucosa of the esophagus. The patients showing the highest mTHPC fluorescence signal had also the highest response to PDT. Also, a correlation between the mTHPC level in the oral cavity and esophagus mucosa has been found. CONCLUSION: PDT can be improved by measuring the mTHPC level in the esophagus or the oral cavity before treatment by fluorescence spectroscopy, and then by adjusting the light dose to be applied to the observed mTHPC level.