In vivo spectrofluorometric analysis during photodynamic therapy (PDT) is a tool to obtain information about fluorophore bleaching kinetics in tissue. Using a cylindrical esophageal light distributor for PDT with an integrated sensing fiber, together with a fluorescence detection setup, we can obtain tissue fluorescence spectra endoscopically in a clinical environment. This study was performed on patients with early squamous cell carcinomas in the esophagus. Patients were injected intravenously with 0.15 mg/kg of mTHPC and underwent PDT (lambda equals 514 nm, fluence rate equals 100 mW/cm 2) 96 hours after injection. Bleaching kinetics of mTHPC and tissue autofluorescence at different wavelengths were recorded in real time and showed decreases in the observed fluorescence intensity in the 652 nm band of about 60% for light doses around 100 J/cm 2. Additional information on bleaching kinetics induced at another excitation wavelength, lambda equals 652 nm, was obtained by irradiations at much lower doses in the buccal cavity. The data are analyzed using a simplified mechanism in which singlet oxygen is the hypothetical reactive intermediate which can both bleach the mTHPC and the autofluorescent molecules. The differential equations are solved by applying the quasi stationary state approximation for the reactive intermediate. The experimental data at least do not appear to contradict this oversimplified mechanism.