To address the significance of lobular neoplasia (LN) in breast core needle biopsy specimens, we prospectively obtained LN cases and correlated results of subsequent tissue sampling. LN was diagnosed by core needle biopsy in 467 women; in 101 (21.6%), invasive carcinoma (IC) or ductal carcinoma in situ (DCIS) was diagnosed concurrently. Two patients (0.4%) had previous diagnoses of IC or DCIS, and 17 (3.6%) had a concurrent diagnosis of contralateral IC or DCIS. Of 366 patients without a concurrent diagnosis of IC or DCIS, subsequent tissue diagnoses were available for 156 cases (42.6%). Of 60 cases of LN and atypical ductal hyperplasia on the biopsy, 5 had IC and 10 had DCIS on the excision (total, 25%). Of 4 women with LN and a mucocele-like lesion on the biopsy, none had IC or DCIS on excision. Of 92 with LN alone on the biopsy, 7 had IC (6) or DCIS (1) on excision. Two cases were in sites away from the biopsy site, 3 in subsequent excisions of the biopsy site, and 2 after previous excision of the biopsy site without finding IC or DCIS. Although LN is associated with a high overall rate of IC and DCIS (30%), excision of the biopsy site for women with LN alone on core needle biopsy has a very low rate of IC and DCIS in our center. Women in whom biopsy sites are excised are still at risk for subsequent DCIS and IC.