The authors retrospectively reviewed 15 cases of pelvic soft tissue sarcoma treated at the Children's Hospital of Philadelphia during a recent 6-year period. These patients were treated before the availability of magnetic resonance imaging. The authors compared ultrasonography, computed tomography, and excretory urography/voiding cystourethrography for establishing the diagnosis and following the progression/regression of the disease. Results were correlated with clinical, surgical, and pathologic findings. Although computed tomography and ultrasonography were both capable of characterizing the size and texture of the lesion, computed tomography was superior in defining disease arising from the pelvic sidewall. Both techniques effectively identified residual mass at the tumor site, but neither could differentiate active tumor from inactive tumor, posttherapy inflammation, or fibrosis. Excretory urography/voiding cystourethrography provided no information that could not be gleaned from either ultrasonography or computed tomography.