We examined 155 patients who received a first-line modified FOLFOX6 ( mFOLFOX6) regimen for unresectable or recurrent colorectal cancer regarding oxaliplatin-related allergic reactions and prognosis of patients who developed such allergic reactions. Twenty eight patients (18.1%) developed a total of 44 allergic reactions (2.9%): The first allergic reactions were grade 1 in nine, grade 2 in seventeen, and grade 3 in two. The median number of cycles of mFOLFOX6 until the first allergic reaction was 8.5 (range, 1-25). Compared with non-allergic group patients (n=127), allergic group patients (n=28) had mFOLFOX6 regimen more frequently (p=0.01) and higher rate of introduction to second-line FOLFIRI regimen (p<0.01). When analysis was restricted to patients who were given a second-line FOLFIRI regimen, the response rate and disease control rate associated with FOLFIRI treatment did not differ significantly between the two groups. However, the allergic group patients tended to show longer disease-free survival (p=0.16) and showed longer overall survival after the start of second-line treatment (p=0.03). In addition, the allergic group patients showed a longer overall survival after the start of mFOLFOX6 (p=0.03). These results suggest that allergic reactions in the first-line mFOLFOX6 treatment might not be a poor prognostic factor, considering the efficacy of second-line FOLFIRI regimen.