Photodynamic therapy is a cancer treatment that combines the effects of oxygen, red light and a photosensitizing dye. The red light is absorbed by dye molecules within the target tissue and the light energy is transferred to molecular oxygen, creating highly reactive oxygen molecules that oxidize tissue components. Clinical results are quite variable because the amount of dye that accumulates in target tissues varies widely between patients. A non-invasive and rapid method to determine the dye content at the time of treatment would allow the light dose to be optimized and may significantly increase the effectiveness of the treatment. I have worked with Herbert Wolfsen, MD (Department of Gastroenterology & Hepatology, Mayo Clinic, Jacksonville) to develop a fluorescence-based method that utilizes a portable fiber optic spectrometer to quantify the Photofrin content in patients undergoing photodynamic therapy for esophageal cancer. I will present an introduction to photodynamic therapy and I will discuss the development of the fluorescence method with clinical results to date.