Summary Background Several studies have shown the efficacy, tolerability, and ease of administration of pemetrexed—an antifolate antineoplastic agent—in patients with advanced non-small-cell lung cancer. We assessed pemetrexed as maintenance therapy in patients with this disease. Methods This randomised double-blind study was undertaken in 83 centres in 20 countries. 663 patients with stage IIIB or IV disease who had not progressed on four cycles of platinum-based chemotherapy were randomly assigned (2:1 ratio) to receive pemetrexed (500 mg/m2 , day 1) plus best supportive care (n=441) or placebo plus best supportive care (n=222) in 21-day cycles until disease progression. Treatment was randomised with the Simon and Pocock minimisation method. Patients and investigators were masked to treatment. All patients received vitamin B12 , folic acid, and dexamethasone. The primary endpoint of progression-free survival and the secondary endpoint of overall survival were analysed by intention to treat. This study is registered with ClinicalTrials.gov , number NCT00102804. Findings All randomly assigned participants were analysed. Pemetrexed significantly improved progression-free survival (4·3 months 95% CI 4·1–4·7 vs 2·6 months 1·7–2·8; hazard ratio HR 0·50, 95% CI 0·42–0·61, p<0·0001) and overall survival (13·4 months 11·9–15·9 vs 10·6 months 8·7–12·0; HR 0·79, 0·65–0·95, p=0·012) compared with placebo. Treatment discontinuations due to drug-related toxic effects were higher in the pemetrexed group than in the placebo group (21 5% vs three 1%). Drug-related grade three or higher toxic effects were higher with pemetrexed than with placebo (70 16% vs nine 4%; p<0·0001), specifically fatigue (22 5% vs one 1%, p=0·001) and neutropenia (13 3% vs 0, p=0·006). No pemetrexed-related deaths occurred. Relatively fewer patients in the pemetrexed group than in the placebo group received systemic post-discontinuation therapy (227 51% vs 149 67%; p=0·0001). Interpretation Maintenance therapy with pemetrexed is well tolerated and offers improved progression-free and overall survival compared with placebo in patients with advanced non-small-cell lung cancer. Funding Eli Lilly.