PURPOSE This phase II study (S0341) evaluated the efficacy and tolerability of single-agent erlotinib in unselected chemotherapy-naive patients with advanced non-small cell lung cancer (NSCLC) and a performance status (PS) of 2. Exploratory analyses of a number of biomarkers relating to epidermal growth factor receptor pathway activation were also performed. PATIENTS AND METHODS Patients with stage IIIB (pleural effusion) or stage IV NSCLC with a PS of 2 and no prior chemotherapy or biologic treatment for NSCLC received erlotinib 150 mg daily. RESULTS A total of 81 patients entered the study; 76 were assessable. One complete and 5 partial responses were noted for an overall response rate of 8% (95% CI 3%-16%). Stable disease (SD) was seen in 26 patients (34%) resulting in a disease control rate (DCR = CR/PR/SD) of 42%. Progression free and median survival were 2.1 months (95% CI 1.5-3.1) and 5 months (95% CI 3.6-7.2), respectively. One-year survival was 24% (95% CI 15%-34%). Although treatment was generally well tolerated, grade 3 to 4 toxicity was reported in 30 patients (40%), including fatigue (16%), rash (9%), diarrhea (7%), and anorexia (7%). There was one possible treatment related death (pneumonitis). CONCLUSIONS In chemotherapy-naive patients with advanced NSCLC and a PS of 2, single agent erlotinib resulted in an acceptable but significant level of treatment-related side effects. With an overall DCR of 42% and median survival of 5 months, results are comparable to those achieved with chemotherapy in this population. Development of an epidermal growth factor receptor-directed biomarker selection strategy may optimize use of erlotinib in PS 2 patients.