Ultrasound findings in 25 patients with surgically proven endometriosis are presented. Of the 31 lesions characterized at ultrasound examination, 17 were described as cystic, four as polycystic, five as mixed, and four as solid. Nine patients had a diagnostic pattern of sonolucent zones within the uterus representing blood lakes (adenomyosis) associated with extrauterine masses. In the remaining 16 patients, ultrasound alone could not differentiate endometriosis from diseases such as tubovarian abscess, ruptured ectopic pregnancy, ovarian cyst(s), or tumor. Clinical history contributed to proper diagnosis in these patients.