To date, this is the first report to monitor changes of intratumor vascularization and the response to radiation and Cyberknife therapy in a patient with recurrent primary papillary serous carcinoma of the peritoneum by three dimensional (3D) power Doppler ultrasonography (PDUS). Transvaginal 3D PDUS detected a recurrent presacral tumor with abundant intratumor vascularity. Serial examinations of the tumor volume and serum CA-125 level were studied before, during, and 6 mo after therapy. Meanwhile, the intratumor blood flow was measured and expressed as vascularity indices. All of the tumor volume, intratumor vascularity indices and serum CA-125 level decreased progressively following therapy. A remaining lesion with nearly absent intratumor power Doppler signals suggested a scarring lesion posttreatment. Indeed, CT-guided tissue biopsy confirmed fibrotic change. 3D PDUS is useful to monitor the response to treatments and to differentiate residual tumors from lesions of scarring change posttreatment. It provides more accurate posttreatment information than pelvic computed tomography.