Cancer has been attributed to 3 causes: pollution, infection, and poor nutrition. Conventional treatments include surgery, chemotherapy, and radiotherapy. The author proposes that immunotherapy also be considered. Among other environmental influences, dietary deficiencies and carcinogenic viral infections must be investigated and treated wherever possible. It has been suggested that mushrooms, in particular, have a structure that is immunomodulatory because it resembles the proteoglycan structure in the human extracellular matrix, and both are metabolically active. Inasmuch as mitochondria have a bacterial origin, proteoglycans may have a mushroom origin. The author describes a study which shows that natural killer cells can double in number with 8 wk of treatment with Coriolus versicolor. Also described is an epidemiological survey of cancer deaths among Flammulina velutipes farmers in Japan, which found that the mushroom farmers had lower rates of cancer deaths than controls who were not involved in mushroom farming.