Pemetrexedis a key drug in the first and second -line therapy for non-small-cell lung cancer. It exhibits an increased area under the plasma drug concentration-time curve, and it has a prolonged half -life when administered to patients with reduced renal function, resulting in a high frequency of neutropenia. Accordingly, pemetrexed is administered to these patients with caution. Herein, we retrospectively investigated the background characteristics of patients with a creatinine clearance rate (Ccr) of<45 mL/min, who experienced severe adverse events due to pemetrexed. Thirty-eight patients with a Ccr of <45 mL/min were administered pemetrexed. Of these patients, 13 (34%) developed severe adverse events (≥Grade 3) such as neutropenia, thrombocytopenia, and nausea. Multiple logistic regression analysis revealed that a Ccr of <30 mL/min (p= 0.033) and the concomitant use of non-steroidal anti-inflammatory drugs (p=0.012) were significant risk factors for adverse events. Therefore, whenever possible, pemetrexed administration should be avoided in patients with a Ccr of <30 mL/ min and in those receiving concomitant non-steroidal anti-inflammatory drugs.