Purpose. The purpose was to evaluate the mammographic and sonographic (US) features of lobular carcinoma in situ (LCIS).Methods. Mammographic and US findings of nine lesions diagnosed pathologically as pure LCIS were analyzed retrospectively according to the American College of Radiology breast imaging reporting and data system (BI-RADS) lexicon.Results. With regards to mammographic findings of LCIS, there were no lesions demonstrated in six cases and a mass in three cases, two of which contained microcalcifications. The most common US findings of LCIS were irregular shape (five cases), ill-defined margins (eight cases), and hypoechogenicity (seven cases). All cases had an elongated shape parallel to the skin or were round (no lesion had a taller-than-wide shape). Two cases were associated with microcalcifications. The final BI-RADS categories were category 3 (probably benign finding) in one case, category 4A (low suspicion of malignancy) in two cases, and category 4B (intermediate suspicion of malignancy) in six cases.Conclusions. LCIS is frequently mammographically occult, and an incidental finding on routine screening mammograms, usually because of microcalcifications. LCIS, a high-risk lesion, can mimic invasive carcinoma on US.