Pemetrexed is established as first-line treatment with cisplatin for malignant pleural mesothelioma and advanced nonsquamous non-small-cell lung cancer (NSCLC) and as single-agent second-line treatment for nonsquamous NSCLC. Because the structure and pharmacokinetics of pemetrexed are similar to those of methotrexate, and methotrexate is associated with severe toxicity in patients with third-space fluid (TSF), the safety of pemetrexed in patients with TSF was evaluated.