Thirteen patients with surgically proven endometriosis underwent ultrasonography as part of their diagnostic evaluation. An attempt was made to assess the sonographic characteristics of endometriosis to determine if a characteristic pattern would emerge when the clinical presentations and physical findings were coupled with the ultrasonic picture. Ultrasound showed pelvic masses in all patients. Most masses appeared spherical and separate from the uterus and were cystic or predominantly cystic with scattered peripheral echoes. This retrospective study suggests a more useful role for ultrasound than previously recognized in the clinical management of patients with endometriosis.