The scientific and anecdotal information available suggests that broken cell wall preparations and extracts of Chlorella pyrenoidosa in the diet provides important nutrients necessary to maintain good health, resist infection, and restore immune functions in immunocompromised patients. Our clinical study of Chlorella pyrenoidosa (provided by YSK International Corporation, Kyoto, Japan) was designed to test these properties in patients with a history of glioma. Patients with this type of primary brain tumor have a poor prognosis and either because of the tumor itself or due to the treatments applied to kill it, they have compromised immune functions leading to a greater than normal susceptibility to infection. Chlorella was added to the diet of 21 patients; 15 had a glioblastoma, four with low-grade astrocytoma, one had an anaplastic astrocytoma, and one with a high-grade oligodendroglioma. Each patient's general health was monitored by monthly physical and neurological examinations as long as they survived or for at least 2 years. Imaging studies of the brain and blood tests were performed at 3–4 months intervals. Complete blood counts, differentials, cytometric determinations of natural killer cells and T cell subsets, and in vitro lymphocyte activation assays to assess level of immunosuppression were performed on blood samples. Our results with patients taking Chlorella, although limited, suggest that the cellular components and functions of the immune system remain at near-normal levels and are less adversely affected when patients are undergoing chemotherapy and/or taking immunosuppressive medications such as steroids. Over the 2 year course of our investigation, we have observed our patients to have had fewer respiratory infections and flu-like illnesses. 7 of the 20 evaluable patients were alive and had yet to show any reappearance of their glioma after 2 years in the study. Although, our results suggest that our glioma patients may have benefited from adding Chlorella pyrenoidosa to their diet, more controlled clinical studies and animal experiments will be required to define its mechanism of action and limitations.