Considered a gourmet mushroom, Maitake is also an important medicinal mushroom in Oriental medicine. It is especially known for its high content of beta-glucans [71, 72], which are known as potent immune boosters.
Maitake is the Japanese name for the mushroom species Grifola frondosa. American mushroom hunters know it as Hen-of-the-woods. Common in eastern Canada and the United States down to the Mid-Atlantic States, Hen-of-the-woods is one of the most easily recognizable wild gourmet mushrooms.
Maitake extract has been found to increase the so called tumor-necrosis-factor [73, 74, 75] as well as other cancer fighting mechanisms. [76, 77, 78, 79, 80, 81] Specific cancer types studied include colorectal, prostate, liver, lung and breast.  FDA has approved a Phase II study of Maitake extract on breast and prostate cancer. 
Other studies on Maitake extract have focused on its immune enhancing properties in general, [84, 85, 86] or on immune-enhancement on cancer patients in specific.  Maitake’s anti-viral properties have received particularly intense scrutiny by the scientific community  and research on Maitake in the treatment of HIV is ongoing. 
Another area of potential interest for Maitake extract may be diabetes type II. One study reported blood glucose decrease of 25% in insulin-resistant individuals. [90, 91, 92]
Additional research has also been conducted on beta-glucan polysaccharides as isolated compounds. There is a pending patent on “Methods of using beta glucan as a radio-protective agent,” which “relates to methods for treating and preventing radiation and/or chemotherapy related injury and/or afflictions, such as myelosuppression and decreased macrophage activity,”  In light of this, it should come as no surprise that NASA gives beta-glucans to its astronauts as protection against radiation poisoning and subsequent immune system degradation.
Other therapeutic effects of Maitake extract that have been researched include: anti-fungal (Candida albicans) and anti-bacterial, blood pressure moderator, nerve tonic, respiratory conditions, and stress reduction. 
Medicinal mushroom expert Paul Stamets, in his book Mycelium Running, also lists Maitake extract as showing some positive effect on leukemia. 
Note: The statements on this page have not been evaluated by the Food and Drug Administration. This article is not intended to diagnose, treat, cure or prevent any disease. Always consult a licensed medical practitioner before using any herb (or mushroom) for medicinal purposes.
Credit to Paul Stamets for research and source material.