Background: A polysaccharide extract from Grifola frondosa (Maitake extract) has shown immunomodulatory effects in preclinical studies and potentials for anticancer immunotherapy. However whether its oral administration in human is associated with measurable immunological change is unknown. Methods: In a phase I/II dose escalation trial, thirty postmenopausal breast cancer patients free of disease after initial treatment were enrolled sequentially into five cohorts of six patients each. Maitake extract was taken orally at 0.1, 0.5, 1.5, 3, or 5 mg/kg twice daily for three weeks. Peripheral blood was collected at day -7, 1 (prior to the first dosing), 7, 14, and 21 for ex vivo analyses. The primary endpoint was safety and tolerability. Results: No dose-limiting toxicity was encountered. Pharmacodynamic activities of the study agent did not follow a linear dose curve, as often seen with immunomodulatory agents. The following correlative endpoints showed significant changes compared to baseline. Quadratic dose curve analysis showed that a daily dosage of 2 mg/kg was associated with the greatest increases of CD3+CD25+ or CD4+CD25+ T cells in the peripheral blood (300% of baseline, p<0.001); a daily dosage of 6 mg/kg was associated with the most significant increases in intracellular IL-2 production by NK-T cells (237% of baseline, p=0.002); IL-10 production by T cells (360% baseline, p=0.002). Interferon gamma production by memory CD4+ T cells was attenuated to 27% of baseline at a daily dose of 7.4 mg/kg (p=0.002). Conclusions: Oral administration of a polysaccharide extract was associated with measurable changes in peripheral blood. The dose associated with the most significant changes varies by immunological parameter. The dose of 6 mg/kg is selected as the dose in future studies with clinical endpoints.