The performance of a novel bronchoscopic fluorescence imaging system was compared with conventional white light bronchoscopy with a data base of 328 biopsy-confirmed sites from 53 patients and 41 volunteers. The two methods were found to have the same specificity (94%); however, the sensitivity of the fluorescence system (72.5%) was found to be 50% greater than that of the white light bronchoscopy (48.4%) in detecting dysplasia and carcinoma in situ. The fluorescence system uses a nonlinear discriminant function combining the red and green image intensity values to form a pseudoimage that, when displayed on an RGB monitor, allows the detection and delineation of abnormal areas. In 15% of the patients with lung cancer, synchronous carcinoma in situ was found in addition to the large invasive cancer. Of the current smokers in this study, 40% had moderate dysplasia and 12% had severe dysplasia. For the ex-smokers 25% had moderate dysplasia, 6% had severe dysplasia, and 13% had carcinoma in situ. Fluorescence imaging may become an important adjunct to conventional bronchoscopic examination to improve our ability to diagnose and stage lung cancer more accurately.