Purpose : The purpose of this study is to evaluate mammographic findings of invasive ductal carcinoma(IDC) andinvasive lobular carcinoma(ILC)and to find differential points between the two. Materials and Methods : 239patients, who underwent mammography prior to surgery and were proved to have IDC(224 patients) or ILC(15 patients)pathologically, were analized retrospectively. On mammogram, presence of mass and microcalcification wereanalized. When there was a mass on mammogram, lesion opacity was classified into high, equal, or low opacity andborder of the mass was classified into spiculated, poorly marginated, and well-marginated. When there was nodefinite mass, mammographic findings were classified into asymmetric opacity and no mass. Results : Masses wereobserved in 168 patients(75%) of IDC and 12 patients(80%) of ILC. Border of the masses were spiculated(n=50,22.3%), poorly marginated(n=112, 50%), or well-marginated(n=6, 2.7%) in patients with IDC. Spiculated and poorlymarginated borders were observed in 8 patients(53.3%) and 4 patients(26.7%) respectively, in patients with ILC.Microcalcifications were seen in 88 patients(39.3%) of IDC and 2 patients(13.3%) of ILC. Equal or low opacities ofthe lesions were observed in 29 patients(17.3%) of IDC and 5 patients(33.3%) of ILC. Conclusion : Although equalor low opacities were observed more frequently in ILC and microcalcifications were noted more frequently in IDC,it was difficult to differentiate the two diseases based on mammographic findings.