Objective: To determine factors predicting the persistence of sonographically identified adnexal masses in pregnancy. Methods: All patients from March 1988 to April 1993 diagnosed with an adnexal mass by obstetric sonography were reviewed. Examinations had been entered prospectively into our sonography database. Follow-up data were collected from the database, from hospital and pathology department records, and from interviews with referring obstetricians. Adnexal masses were characterized by size, sonographic appearance, and anatomic site. Persistence of the masses was determined by subsequent sonography, operative findings, or postpartum physical examinations. Results: The rate of adnexal masses during pregnancy was 2.3% (432 of 18,391). Complete follow-up was available for 422 of 432. Most of the adnexal masses (76%; 320 of 422) were simple cysts with a mean diameter less than 5 cm. The remainder of the masses were simple or complex, measuring 5 cm or more in diameter. Seventy of 102 large or complex masses resolved. By multivariate analysis, the best predictors for persistence of these masses were complex appearance on sonography and size of the mass (P < .05 for both categories). Conclusion: Most adnexal masses identified by sonography during pregnancy were small, simple cysts that did not pose a risk to the pregnancy. Even the majority of large or sonographically complex masses resolved. The best predictors of persistence of the masses were sonographic appearance and size.