Solidago virgaurea (goldenrod) has traditionally been used as an anti-inflammatory herbal medicine for the treatment of various symptoms, including prostatic diseases. The plant has also been reported to have antibacterial, spasmolytic, and carminative properties. During the course of our screening for antineoplastic activities in various herbal plants, we found that the extract of S. virgaurea exhibits strong cytotoxic activities on various tumor cell lines. The active component mostly resides in the leaves of the plant and is soluble in water. When the extract was fractionated by a Sephadex G-100 column, the active fraction corresponded to a molecular weight of approximately 40,000. This cytotoxic activity is effective on various tumor cell lines, including human prostate (PC3), breast (MDA435), melanoma (C8161), and small cell lung carcinoma (H520). To examine the effect of the cytotoxic activity on tumor cells in vivo, we used the rat prostate cell line (AT6.1) and an SCID mouse model. AT6.1 cells were injected into the flank of SCID mice, and then the G-100 fraction of S. virgaurea was administered intraperitoneally or subcutaneously every 3 days. The size of the tumor was measured for up to 25 days. The growth of the tumor was significantly suppressed by the G-100 fraction at 5 mg/kg without any apparent side effects. Therefore, S. virgaurea is considered to be promising as an antineoplastic medicine with minimal toxicities.