Specially developed high-resolution real-time ultrasound transducers (12.5 and 20 MHz) on the tip of endoluminal catheters were inserted into the endometrial canal to evaluate the usefulness of this approach. Uterine abnormalities, most confirmed by biopsy, surgery, or both, were detected in 12 patients, including submucosal myomas, nabothian cysts, endometrial polyps, synechiae, and endometrial and cervical carcinoma. In one case the catheter was directed under hysteroscopic guidance into a fallopian tube, demonstrating its potential usefulness in this region. In 4 of the 12 cases in which a hysterectomy was performed, an in vitro ultrasound examination of the organ was performed, which confirmed the initial in vivo ultrasound impressions. Anatomic cross-sectional slices of the uterus resulted in excellent correlation with the ultrasound findings. These preliminary results suggest that this new sonographic procedure will become an important diagnostic tool, supplementing abdominal and endovaginal ultrasound approaches.