Although S-1 has been shown to have activity against advanced nonsmall-cell lung cancer (NSCLC), its efficacy for elderly patients remains unclear. This phase II study evaluated the efficacy and safety of S-1 as a first-line treatment for elderly patients. Chemotherapy-naïve patients aged 70 years or older with stages IIIB to IV or postoperative NSCLC and performance status 1 or lower were eligible. Patients received S-1 approximately equivalent to 80 mg/m/day for 2 weeks followed by a 1-week rest period every 3 weeks. The primary end point was the response rate. Secondary end points were toxicity, disease control rate, progression-free survival, and overall survival. Twenty-nine patients were eligible. The median age was 78 years (range, 70-85 years). The overall response rate and the disease control rate were 27.6 [95% confidence interval (CI), 11.3-43.9%] and 65.5% (95% CI: 48.2-82.8%), respectively. The median progression-free survival time was 4.0 months (95% CI: 4.0-9.8 months). The median overall survival was 12.1 months (95% CI: 13.8-25.5 months) and the 1-year survival rate was 53.6%. No grade 4 toxicities were observed. The only hematological toxicity of grade 3 was anemia in 6.9% of patients. The grade 3 nonhematological toxicities included hyponatremia, anorexia, nausea, oral mucositis, and diarrhea in 3.4% of patients and infection in 6.9% of patients. S-1 monotherapy was effective and well tolerated as a first-line treatment for elderly patients with advanced NSCLC. The results of this study warrant further investigations of this regimen, including a randomized controlled trial.