We had cases in which peripheral neuropathy was augmented after changing from mFOLFOX6, a chemotherapy for colorectal cancer, to FOLFIRI and comparatively examined disease status and trends. There were no shared points with respect to patient characteristics, timing of peripheral neuropathy augmentation, drug dosage, etc. It appeared that the change in chemotherapy itself had an effect on neuropathic symptoms. Regarding the change in chemotherapy, the therapeutic agent was switched from oxaliplatin to irinotecan; the cause was unknown, but some effects of these two drugs were suggested. Future investigation, including the examination of genetic mutations, is necessary.