Active Hexose Correlated Compound (AHCC) is an extract of Lentinula edodes of the basidiomycete family of fungi rich in alpha glucans. AHCC has been used for many years as a dietary supplement to enhance the immune system and in clinical trials as an adjunctive treatment in Hepatocellular cancer. This multiple dose, Phase I trial, using FDA guidelines, directly investigates the clinical safety and tolerability of AHCC in healthy subjects. Its safety has been based previously on anecdotal reports and its use in clinical practice. Twenty-six healthy male or female subjects between the ages of 18 and 61 were recruited from the community and gave their consent to participate in the trial. The subjects were given 9 g of AHCC (150 mL of the currently available liquid AHCC) PO daily for 14 d. Laboratory data was obtained at baseline and after 14 d of exposure to AHCC and adverse events were monitored by a non-directed review of systems questionnaire three times during the trial. At each visit the vital signs and adverse events were recorded. Two subjects (7%) dropped out because of nausea and intolerance of the liquid. Adverse effects of nausea, diarrhea, bloating, headache, fatigue, and foot cramps occurred in a total of 6 subjects (20%) but were mild and transient. There were no laboratory abnormalities. When used in high dose in healthy subjects, AHCC causes no significant abnormality in laboratory parameters. The adverse effects of 9 g of liquid AHCC per day, a higher dose than used in routine clinical applications, are minimal and the dose was tolerated by 85% of the subjects.