We studied the role of color Doppler ultrasonography in the distinction between endometriomas and other adnexal masses. Three hundred and fifty-two ovarian lesions were studied, comparing sonographic diagnosis with pathologic findings. On color Doppler sonography, an endometriotic cyst usually appeared as a cystic lesion with diffuse internal echoes and low vascularization. The sensitivity and specificity of color Doppler transvaginal sonography in detecting endometriotic cysts were 91.8% and 95.3%, respectively. The positive and negative predictive values were 95.5% and 91.5%, respectively. In our experience, transvaginal sonography with color Doppler interrogation is a useful technique in the diagnosis of pathologic ovarian conditions, including cystic endometriosis.