We sought to determine the accuracy of sonographic diagnosis and perinatal outcome in pregnancies with maternal adnexal masses. All patients with a prenatal sonographic diagnosis of an adnexal mass measuring 4 cm or greater beyond 12 weeks of gestation were identified. Only masses associated with live fetuses, for which pathologic follow-up or direct visualization of the ovary was available at a single hospital, were included in the study. Sonographic evaluation of the pelvic mass was done prospectively at the time of the original scan and characterized on the basis of morphologic criteria. One hundred and twenty-five pregnant patients 21 to 47 years old with 131 lesions formed the study group. The pathologic diagnoses included 40 dermoids, 15 endometriomas, 14 cysts, 13 cystadenomas, nine tubal cysts, four fibroids, and one ovarian cancer. Six patients had complex pathologic lesions and five had unusual diagnoses. Twenty-four patients had normal ovaries on follow-up examination. Sonographically benign appearing lesions were seen in 89.3% of patients. Ninety-five percent of dermoids, 80% of endometriomas, and 71% of simple cysts were characterized correctly. Fourteen of the 131 lesions (10.7%) had sonographic characteristics suggestive of malignancy. One of these 14 patients (7%) had ovarian cancer. This represents a 0.8% malignancy rate among the total number of lesions. Twenty-four of the 125 patients (19%) underwent second trimester laparotomy at the discretion of their managing obstetricians, with no pregnancy losses. One patient had acute torsion of a dermoid at 39 weeks. Prenatal sonography can accurately characterize maternal adnexal lesions.