Despite the expectation that tumor blood flow information obtained by color Doppler sonography would be useful in diagnosiing malignant ovarian tumors, reported results are not always in agreement. We used definite criteria to study the usefulness of color Doppler sonography in diagnosing a large number of cases of malignant ovarian tumor. Here we analyze the effectiveness of blood flow and imaging information intended for use in detecting ovarian malignancy. Eighty-two (72 cases) of the 308 adnexaltumors (282 surgically proven cases) examined by sonography and color Doppler at this institution during the 62-month period starting in April1992 were malignant. Accuracy in diagnosing malignant ovarian tumors, differentiated by evaluating sonograms according to the classification system proposed by the Japan Society for Ultrasonics in Medicine, showed 87.8% sensitivity 77.4% specificity, positive predictive value of 58.5% and negative predictive value of 94.6%. Blood flow analysis carried out using color Doppler sonography revealed that depiction rate of intratumoral blood flow was significantly higher in the malignant (91.5%, 75 of 82 tumors) than in the benign tumors (18.6%, 42 of 226 tumors). Further, blood flow was detected most frequently (60.0%) in the solid portion of the malignant tumors, less frequently in the septum (37.3%), and least frequently in the outer wall (2.7%). An interrupted and/or irregular type I blood flow pattern was more likely to appear in malignant tumors than were the regular and/or continuous type ュカ patterns. RI was significantly lower in malignant than in benign tumors, irrespective of flow pattern or location. A cut-off value of 0.5 was superior to one of 0.4 or 0.6 in differentiating malignancy. Neither PSV nor ATI warranted use as a diagnostic indicator of malignancy. Our results indicate that color Doppler sonography can detect several characteristics of blood flow in malignant tumors, tumors and in particular, blood flow of low impedance (low RI). We thus suggest that supplementary use of the color Doppler method would improve the accuracy of ultrasound diagnosis of ovarian malignancies.