STUDY OBJECTIVE: To estimate the diagnostic performance of magnetic resonance imaging (MRI) in detection of myomas and adenomyosis of the uterus.DESIGN: Prospective cohort observational study (Canadian Task Force classification II-2).SETTING: Department of obstetrics and gynecology, tertiary academic hospital.PATIENTS: One hundred fifty-three consecutive women with an enlarged uterus accompanied by gynecologic symptoms and/or with an asymptomatic pelvic mass.INTERVENTION: Total abdominal hysterectomy. All patients underwent MRI before the operation.MEASUREMENTS AND MAIN RESULTS: The sensitivity, specificity, positive, and negative predictive value of MRI for the diagnosis of uterine pathology was calculated using histologic findings as the standard criterion for final diagnosis. Receiver operating characteristics curves were constructed to describe the diagnostic performance of MRI. In the diagnosis of myomas, MRI demonstrated sensitivity of 94.1%, specificity of 68.7%, PPV of 95.7%, and NPV of 61.1%. In the diagnosis of adenomyosis, MRI demonstrated sensitivity of 46.1%, specificity of 99.1%, PPV of 92.3%, and NPV of 88.5%. The area under the curve (AUC) for the diagnostic performance of MRI in the detection of myomas and adenomyosis was 0.81 and 0.73, respectively. Uterine sarcoma was diagnosed in 5 patients; in these cases, MRI demonstrated sensitivity of 60.0%, specificity of 99.2%, PPV of 75.0%, and NPV of 98.4%. The AUC for MRI in the diagnosis of uterine sarcomas was 0.80.CONCLUSIONS: MRI exhibits a high AUC for the diagnosis of both adenomyosis and myomas. The PPV of MRI in the diagnosis of adenomyosis and myomas of the uterus is high as well. MRI seems to be a useful technique in everyday clinical practice in the diagnostic approach of these common conditions, enabling clinicians to select the most appropriate management.