A new immunomodulating agent, bestatin (INN: ubenimex) has low toxicity after long-term oral administration and significantly modifies immunological responses. Prolongation of remission duration and survival was achieved in adult acute nonlymphocytic leukemia with bestatin immunotherapy combined with remission maintenance chemotherapy. Patients with myelodysplatic syndrome (MDS) and chronic myelogenous leukemia (CML) responded to bestatin, and it is noted that cytogenetic remission was obtained in CML. MDS and CML are thought to be a family of clonal malignant disorders in which malignant transformations occurs at the level of the pluripotent stem cell. Bestatin may be capable of modifying the biological-proliferative disequilibrium of the disease, and the therapy opens new and promising prospects in the treatment of both MDS and CML. Randomized controlled studies of bestatin immunotherapy were performed in solid tumors including malignant melanoma, carcinoma of the lung, stomach, bladder, head and neck, and esophagus, and therapeutic benefits on disease free-interval or survival were observed in certain types of these cancers. However, the adjuvant activity of bestatin immunotherapy for these cancers should be further investigated to confirm its activity.