Known for its high content of beta-glucans [71, 72], powerful immune-stimulating polysaccharides, the gourmet mushroom Maitake is one of the most commonly used medicinal mushrooms in Traditional Chinese Medicine (TCM).
The common English name for Maitake is Hen-of-the-woods, due to its occasional resemblance to the fluffed up tail feathers of a hen. It is one of the most easily recognizable edible mushrooms in North America, though it is only to be found in eastern Canada and U.S. down to the Mid-Atlantic States.
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 Maitake research has focused on its ability to enhance the immune system, both in general [84, 85, 86] as well as in cancer patients.  The anti-viral potential of Maitake extract has been of particular interest.  Viruses are, after all, notoriously difficult to treat. Research on Maitake and HIV is being conducted. 
Of further interest is the well demonstrated blood sugar lowering effect of Maitake extract. One study on insulin-resistance found blood glucose drops of 25% average. [90, 91, 92]
In addition to research on Maitake extract, there are also many studies on the individual active compounds, in particular beta-glucans. A patent application exists on "Methods of using beta glucan as a radio-protective agent." The patent in question "relates to methods for treating and preventing radiation and/or chemotherapy related injury and/or afflictions, such as myelosuppression and decreased macrophage activity."  This protective effect against radiation poisoning and degradation of the immune system may be why NASA gives beta-glucans to astronauts.
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: Thank you to Paul Stamets for source material.