The present study aimed to determine the effect of oral ${eta}$-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 ${eta}$-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 ${eta}$-glucan group were $7,300/mm^3$ and $3,800/mm^3$, respectively, and the reductions, as compared to baseline, were not significant (p=0.673 and 0.784). The median platelet count was $264,000/mm^3$ after chemotherapy in the ${eta}$-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 ${eta}$-glucan group. We conclude that ${eta}$-glucan can be used to reduce the adverse effects of chemotherapy.