Pemetrexed, a folate metabolic antagonist, is considered to be effective against plural mesotheliomas, non-small cell lung cancer, and especially for non-squamous cell cancer. However, it has been reported to have adverse interactions with nonsteroid anti-inflammatory drugs(NSAIDs). In the present study, we compared the incidence of adverse events between patients receiving pemetrexed therapy with and without concomitant NSAID administration. No significant difference in the incidence of hematotoxic events of Grade 3 or worse was observed. As for the incidence of non-hematotoxic events, the increase in the amount of creatinine, namely a severe adverse effect of Grade 2 or more, was significantly higher in the combined therapy group (p=0.018). No other significant differences were noted for other adverse events. A creatinine increase to Grade 2 or greater developed significantly earlier in the combined group(median value, 12.7 courses; p=0.0063). Our results suggest that renal dysfunction may easily develop as a result of continued pemetrexed administration combined with NSAID therapy. Therefore, it is necessary to take precautions against adverse side effects such as renal dysfunction when combining pemetrexed with NSAID therapy, by conducting periodic examinations.