[A case of interstitial pneumonitis induced by S-1].

S-1, an oral fluoropyrimidine derivative, has been identified as an effective agent for the treatment of breast cancer. We present here a case of interstitial pneumonitis that occurred after S-1 treatment. A n 80-year-old woman was diagnosed with stage III infiltrating ductal carcinoma of the left breast and underwent a modified radical mastectomy in November 2001, followed by six courses of paclitaxel.