The present study aimed to determine the effect of oral β-glucan on mucositis and leukopenia in 62 consecutive patients with colorectal cancer treated with an adjuvant FOLFOX-4 regimen. The patients were retrospectively evaluated in 2 groups: one group received β-glucan and the other did not (control group). Leucocytes, neutrophils, and platelets were evaluated before and 1 week after chemotherapy and oral mucositis and diarrhea were noted. Leucocyte and neutrophil counts after chemotherapy in the β-glucan group were 7,300/mm3 and 3,800/mm3, respectively, and the reductions, as compared to baseline, were not significant (p=0.673 and 0.784). The median platelet count was 264,000/mm3 after chemotherapy in the β-glucan group and the reduction, as compared to baseline, was borderline significant (p=0.048). In the control group, reduction in leucocyte, neutrophil, and platelet counts was statistically significant. Oral mucositis and diarrhea were less common in the β-glucan group. We conclude that β-glucan can be used to reduce the adverse effects of chemotherapy.