Hematoporphyrin derivative (HpD) administration and laser photoradiation were combined to clarify the system's clinical applicability in localizing lung cancer tumors. It was used in 16 cases of lung cancer and in one case of severely atypical squamous metaplasia. Tumors were irradiated 48 hours after intravenous injection of 2.0 to 4.0 mg/kg HpD and fluorescence observed. Fluorescence was seen in 13 lung cancer cases in the case of squamous metaplasia, but results were negative in three of the cases of lung cancer. Among the positive cases was one of occult cancer and two invading the trachea, difficult to recognize by conventional fiberoptic bronchoscopy, as was the severely atypical squamous metaplasia. In the three lung cancer cases not fluorescing, the tumors were obscured by blood, necrotic tissue, or normal mucosa. The method holds promise in the diagnosis of malignant tumors. Nevertheless, it needs further refinement and more studies to elucidate definitive differentiation between malignant and severely atypical, nonmalignant tissue.