Photofrin is the most commonly used photosensitizer for photodynamic therapy (PDT). The major side effect of Photofrin is cutaneous photosensitivity. A second generation photosensitizer, mono-L-aspartyl chlorin e6 (NPe6) has shown anti-tumor efficacy and rapid clearance from skin. Therefore, we conducted a phase II clinical study to investigate the anti-tumor effects and safety of NPe6 in patients with early superficial squamous cell carcinoma of the lung. Enrollment criteria consisted of endoscopically evaluated early stage lung cancer with normal chest X-ray and CT images, no lymph node or distant metastasis. Tumors were located no more peripherally than subsegmental bronchi, the peripheral margin had to visible, and the tumor size had to not more than 2 cm in diameter. The histologic type of the tumor had to squamous cell carcinoma. Laser irradiation (100 J/cm2) using a diode laser was performed at 4 h after administration of NPe6 (40 mg/m2). Among 41 patients with 46 lesions, 40 with 45 lesions were eligible for safety evaluation, and 35 patients with 39 lesions were judged as eligible for efficacy evaluation. No serious adverse drug reactions were observed. Disappearance of skin photosensitivity was recognized within 2 weeks in 28 of 33 patients (84.8%) and in all the other seven patients first tested at 15-18 days. Complete response (CR) was seen in 84.6% of lesions (82.9% of patients). This study demonstrated excellent anti-tumor effects and safety, especially low skin photosensitivity in patients with early stage lung cancer. PDT using the second generation photosensitizer NPe6 and a diode laser will likely become a standard modality of PDT for central type early superficial squamous cell carcinoma of the lung.