The differentiation of benign from malignant intraductal papillary mucinous tumors (IPMT) is often difficult even by various examination methods. We evaluated the qualitative and quantitative diagnostic ability of contrast-enhanced transabdominal ultrasonography (CE-US), mainly in differentiating benign from malignant tumors in patients with IPMT. There were 21 patients with IPMT who underwent CE-US and endoscopic ultrasonography (EUS). Surgery was performed in all 21 patients. Pathological findings were 4 with carcinoma and 17 with adenoma. CE-US was performed using a contrast agent (Levovist; Tanabe, Osaka, Japan) consisting of galactose microbubbles and a small (0.1%) admixture of palmitic acid, and the following items were evaluated by the following procedure. (1) Two reviewers with experienced sonographic and endosonographic ability evaluated CE-US images before and after contrast enhancement and classified the enhancement effects into three grades. In addition, the presence or absence of enhancement effects by CE-US was compared with that of mural nodules visualized by EUS. (2) In all 21 patients, changes in intensity after contrast enhancement were quantitatively measured using an HDI Lab. HDI Lab was provided by ATL (Philips; Bothell, WA) and these software tools rapidly quantify image characteristics within multiple ROI (regions of interest) and make comparisons between several areas or images. In both the early and late phases, the post-enhancement intensity, difference between pre- and post-enhancement intensity, and the percentage change ((post-enhancement value-pre-enhancement value)/pre-enhancement value) were compared between malignant and benign lesions, and the ability of CE-US to differentiate between benign and malignant lesions was evaluated in comparison with the ability of EUS to diagnose the degree of malignancy. (1) In both the early and the late phases, both reviewers observed enhancement effects in all 21 patients. And both reviewers observed mural nodules by EUS in all 21 patients. (2) In all 21 patients who underwent resection of IPMT, the intensity increased in both the early and late phases. When the patients with carcinoma were compared with those with adenoma, the post-enhancement intensity was significantly higher, and the difference between pre- and post-enhancement intensity and the percentage change in the early phase and the late phase was significantly more marked in the carcinoma group (p= 0.019, p= 0.002, p= 0.015, p= 0.012, and p= 0.039, respectively). CE-US was useful for qualitatively diagnosing tumor lesions in patients with IPMT. Moreover, quantitative changes in intensity can be a parameter for the differential diagnosis of benign and malignant tumors.