TAS-102 was first approved in Japan in March 2014 for the treatment of metastatic colorectal cancer (mCRC) and has demonstrated efficacy in fluorouracil (5-Fu) refractory patients based on having a different mechanism of action [
1]. The safety of TAS-102 with the Japanese recommended dose has also been independently confirmed [
2]. The toxicity profile of TAS-102 differs from the known adverse effects of 5-Fu and its derivatives. In the RECOURSE trial, TAS-102 was associated with a favorable overall safety profile, but treatment was accompanied with a significant increase of hematological toxicities. In particular, 38 % of patients presented with grade 3–4 neutropenia, although febrile neutropenia was only observed in 4 % of the patients [
3]. However, TAS-102 has never been clearly associated with incidences of interstitial pneumonia (IP). According to the phase 2 trial (J003 trial) involving 169 Japanese patients, there was one suspected case of interstitial lung disease (ILD) [
4]. A few reports of IP were reported for TAS-102 in combined therapy with fluorouracil, leucovorin, and oxaliplatin (FOLFOX4) [0.6 % (2 out of 322 patients)] and with fluorouracil, leucovorin, and irinotecan (FOLFIRI) [0.7 % (2 out of 302 patients)] [
5]. Induced ILD has also been reported in 1.3 % (39 out of 3085) of patients receiving the molecularly targeted therapy panitumumab [
6]. There have been no reports of ILD in patients receiving TAS-102 chemotherapy, whereas seven cases were reported from the early post-marketing phase vigilance (EPPV) on TAS-102 in Japan [
7]. Herein, to the best of our knowledge, we report the first case of IP associated with TAS-102 therapy in a patient with mCRC.