We have developed a cell-based immunotherapy and successfully treated some patients with benzene-induced and idiopathic aplastic anemia (AA). Autologous and/or allogeneic peripheral blood mononuclear cells were cultured in vitro with a combination of cytokines and calcium ionophore for 2 days before given to patients via intravenous infusion--initially with patients with mild or modest disease. However, these patients had been treated with conventional immune suppression plus growth factors for 6 months to 1.5 years. During this rather prolonged time, they did not recover spontaneously, and one or more lineages (mostly platelet) of their peripheral blood counts had never been normal. The frequency of the treatment was once a week and patients stopped receiving any other therapy. Encouraged by the fact that these patients had strong and rapid recovery of blood counts after receiving a number of cycles of the therapy, we then treated patients with more severe disease. This immunotherapy is very powerful in that no patients, among total 29 we have treated so far, even with the most severe form of benzene-induced AA, have failed therapy in our hospitals. It was found that severe disease requires more aggressive and prolonged therapy, the longest period of time we have performed the therapy for such patients is one year. The first group of patients we treated were followed up to 2.5 years and no secondary clonal disorders were found. It was found in our ongoing studies that infusion of large numbers of allogeneic immune cells is even more effective than infusion of small numbers of autologous cells for patients with severe disease. We have used 2–5x108 allogeneic immune cells per infusion per day for 5 consecutive days, followed by small numbers of autologous infusions (1 to 10 million from 50 ml of peripheral blood, depending on the severity of the disease). This cycle of therapy is repeated once a month for as long as the neutrophil count is more than 0.5x109/L. The efficacy of the therapy seems to correlate well with the number of cells infused and the frequency of infusion. The mechanism by which our immunotherapy works is not completely clear to us, and we are currently trying to understand it better by performing various animal experiments to see how stem cells benefit from the therapy. Because allogeneic cells work well for patients with AA, we speculate that our immunotherapy has little to do with HLA-mediated specific immune responses but rather affects target cells, tissues, and organs through cytokines produced by both infused and target cells and cell contact. Analysis of patients’ thymus function indicates improvement in T-cell differentiation and maturation. Finally, we are using the same therapy to treat 6 patients with severe idiopathic AA and have found that the therapy is also effective for these patients. One patient had a complete response after 3 months of therapy and the other 5 have had dramatic improvements in bone marrow histology, reduced blood transfusion, and increased neutrophil, leukocyte and reticulocyte counts after several months of therapy. We found, in general, that idiopathic AA is much more difficult and requires an even longer period of time to cure than benzene-induced AA using this immunotherapy and also that cure of this terrible disease using biotherapeutic approachs may become possible. This simple but very effective immunotherapy may have more potential in treatment of other hematopoietic disorders.