Objectives: Anthracyclines and taxanes are often used as first-line chemotherapy treatments in patients with breast cancer. There are, however, significant toxicity and side effects associated with these therapies. Previous studies have demonstrated that active hexose-correlated compound (AHCC) reduces such side effects. The present study explored the beneficial effects of AHCC on adverse events in patients receiving adjuvant chemotherapy for breast cancer. Subjects: Forty-one women who were treated with anthracyclines and taxanes at Nagumo Clinic in Tokyo from October 2004 to March 2011 were selected for this study. Outcome measures: We compared the occurrence of adverse events in patients who received AHCC with those who did not receive AHCC. Using Fisher's exact tests, we also compared the worst-grade adverse events in each treatment cycle. Generalized estimating equations were employed to compare longitudinal changes, and the use of granulocyte colony-stimulating factor, in the two groups was analyzed using Student's t-test. Results: We found that, compared to the control group, the AHCC group had significantly fewer neutrophil-related events (odds ratio, 0.30; p=0.016), significantly lower use of granulocyte colony-stimulating factor, and a higher (although not significant) rate of adverse events associated with γ-glutamyl transpeptidase. Conclusions: AHCC has the potential to reduce the severity of neutropenia induced by breast cancer chemotherapy and the use of G-CSF during chemotherapy.