Hematoporphyrin is preferentially concentrated by cancer tissue and therefore is a potentially useful chemical marker. This report demonstrates the effectiveness of a photoelectric hematoporphyrin fluorescence detector used simultaneously with flexible fiberoptic bronchoscopy as a means of detecting and localizing bronchogenic carcinoma. The detector generates an audio signal when fluorescence is within the visual field of the bronchoscope. Of interest is that an audio signal was generated which indicated hematoporphyrin fluorescence in areas where no mucosal abnormality had been seen. These areas proved to be carcinoma in situ.