Summary Background Activating mutations in EGFR are important markers of response to tyrosine kinase inhibitor (TKI) therapy in non-small-cell lung cancer (NSCLC). The OPTIMAL study compared efficacy and tolerability of the TKI erlotinib versus standard chemotherapy in the first-line treatment of patients with advanced EGFR mutation-positive NSCLC. Methods We undertook an open-label, randomised, phase 3 trial at 22 centres in China. Patients older than 18 years with histologically confirmed stage IIIB or IV NSCLC and a confirmed activating mutation of EGFR (exon 19 deletion or exon 21 L858R point mutation) received either oral erlotinib (150 mg/day) until disease progression or unacceptable toxic effects, or up to four cycles of gemcitabine plus carboplatin. Patients were randomly assigned (1:1) with a minimisation procedure and were stratified according to EGFR mutation type, histological subtype (adenocarcinoma vs non-adenocarcinoma), and smoking status. The primary outcome was progression-free survival, analysed in patients with confirmed disease who received at least one dose of study treatment. The trial is registered at ClinicalTrials.gov , number NCT00874419 , and has completed enrolment; patients are still in follow-up. Findings 83 patients were randomly assigned to receive erlotinib and 82 to receive gemcitabine plus carboplatin; 82 in the erlotinib group and 72 in the chemotherapy group were included in analysis of the primary endpoint. Median progression-free survival was significantly longer in erlotinib-treated patients than in those on chemotherapy (13.1 95% CI 10.58–16.53 vs 4.6 4.21–5.42 months; hazard ratio 0.16, 95% CI 0.10–0.26; p<0.0001). Chemotherapy was associated with more grade 3 or 4 toxic effects than was erlotinib (including neutropenia in 30 42% of 72 patients and thrombocytopenia in 29 40% patients on chemotherapy vs no patients with either event on erlotinib); the most common grade 3 or 4 toxic effects with erlotinib were increased alanine aminotransferase concentrations (three 4% of 83 patients) and skin rash (two 2% patients). Chemotherapy was also associated with increased treatment-related serious adverse events (ten 14% of 72 patients decreased platelet count, n=8; decreased neutrophil count, n=1; hepatic dysfunction, n=1 vs two 2% of 83 patients both hepatic dysfunction). Interpretation Compared with standard chemotherapy, erlotinib conferred a significant progression-free survival benefit in patients with advanced EGFR mutation-positive NSCLC and was associated with more favourable tolerability. These findings suggest that erlotinib is important for first-line treatment of patients with advanced EGFR mutation-positive NSCLC. Funding F Hoffmann-La Roche Ltd (China); Science and Technology Commission of Shanghai Municipality.