Based on confirmation that slightly toxic mumps virus having a strong affinity with mankind has a carcinostatic effect, it was used in cancer. As priority was put not on healing but on reconfirmation of its effectiveness against human cancer and on its mode of administration, patients mostly received only small amounts of the virus. Of 90 patients with terminal cancer of various kinds, treatment was assessed as very good in 37, and good in 42, excluding 11 patients who had been near death. Administration of mumps virus produced few side‐effects. The initial antineoplastic effect of the mumps virus therapy seemed to occur rapidly and strongly in proportion to swiftness in proliferation of cancer cells. Patients retaining physical strength often showed continuously suppressed growth of remaining tumors even after the disappearance of the initial effect, indicating a possible participation of tumor immunity in the present virus therapy.