Endometriomas have a prevalence of 24% among all ovarian cysts. Various sonographic features have been proposed to identify endometriomas. Although the visualization of ovarian masses with low-level internal echoes is suggestive for the endometriotic origin of the cyst, no data are yet available on the specificity of endovaginal ultrasonography in differentiating endometriomas from other ovarian masses. To address this issue, the sensitivity, specificity, negative and positive predictive values of endovaginal ultrasonography in comparison with pathology were calculated for each visualized cyst. The study population (n = 251) consisted of all premenopausal non pregnant women submitted to laparotomy or laparoscopy between May 1991 and March 1993 at the Department of Obstetrics and Gynecology of the University of Cagliari. Within one week before surgery, all patients underwent endovaginal ultrasonography and 93 ovarian cysts were visualized. After the scan, the physician gave prospective impressions as to the presence of endometriomas using the visualization of round-shaped homogeneous hypoechoic "tissue" of low-level echoes within the ovary as characteristic ultrasonographic finding. Ultrasonographic impression was compared with histopathological diagnosis. Out of 93 adnexal masses detected by ultrasound, 31 were suspected to be endometriomas and the diagnosis was confirmed in 24. The sensitivity and the specificity of endovaginal ultrasonography in differentiating endometriomas from other ovarian cysts were 83% and 89%, respectively. This specificity (89%) is comparable with that obtainable with magnetic resonance imaging (91%).