Intrauterine tandem placement can be difficult in brachytherapy for cervical cancer. A prospective study was planned to investigate transvaginal ultrasound (TVU) guidance for intrauterine tandem insertions in the treatment of cervix cancer.Between January 1997 and January 2002, TVU was used after pelvic external beam radiotherapy in 48 cervical cancer patients with a high complication risk because the cervical canal orifice could not be identified and/or the patients could not undergo hysterometry. The TVU findings were graded as 1-4, with a higher number indicating greater difficulty in implantation. The tandem was inserted using the guidance of the TVU findings. Tandem application difficulties were graded in accordance with the number of implantation trials. The relations between tandem applicability and possible factors were analyzed.By TVU, the difficulties in tandem insertion were graded as Grade 1 in 5 cases, Grade 2 in 23 cases, Grade 3 in 17 cases, and Grade 4 in 3 cases. The tandem was inserted in 19 patients on the first try, 12 patients on the second try, 12 patients on the third try, and 2 patients on the fourth try. However, the cervical canal could not be opened in 3 patients. Factors such as median age, stage, chemotherapy, diameter of the tandem applicators, time between external beam radiotherapy and brachytherapy, and the rate of tumor regression were not statistically significant. The ultrasound grade (p = 0.02) and diameter of the tandem applicator (p = 0.007) were statistically significant. Perforation and sepsis were not observed.TVU guidance before brachytherapy has a positive contributory effect in reducing the morbidity rate of patients considered high risk.