Lobular neoplasia (LN), including atypical lobular hyperplasia (ALH) and lobular carcinoma in situ, may be encountered in breast core biopsies performed for mammographic abnormalities even though LN is often not, in itself, responsible for the abnormal mammogram. The need for surgical excision following a diagnosis of LN on core biopsy is not well defined. We examined pathologic and mammographic findings in a consecutive series of cases diagnosed as LN to address this issue. Radiology/pathology records were reviewed for cases with a pathology diagnosis of pure LN during the period 1998-2001. Specifically excluded were cases with associated atypical ductal hyperplasia, ductal carcinoma in situ, invasive mammary carcinoma, or any history of breast malignancy. Thirty-five women 39-76 years of age (mean 52 years) were identified. Specimens were obtained as stereotactic core (31) or limited wire-guided biopsy (four). The diagnoses were lobular carcinoma in situ (12), lobular carcinoma in situ/ALH (10), and ALH (13). Fourteen patients did not undergo excisional biopsy and had no subsequent clinical follow-up to warrant additional biopsy (follow-up 6 months to 3 years). Five patients had no immediate excision, but eventually during clinical follow-up for LN (1 month to 3 years), two developed mammographic lesions in the ipsilateral (one patient) or contralateral breast (one patient) that led to diagnoses of invasive mammary carcinoma (lobular and composite ductal-lobular types, 10 and 8 mm, respectively); three patients had subsequent mammographic findings in the ipsilateral or contralateral breast leading to biopsies showing only LN (two patients) or no neoplastic pathology (one patient). The remaining 16 patients (all core biopsied) underwent immediate wire-guided excisions. Thirteen (81%) showed additional foci of LN, one (6.3%) with atypical ductal hyperplasia, and two (12.5%) with invasive lobular carcinoma (3 mm and <1 mm). Three (19%) had no residual disease; however, additional clinical follow-up in one of these patients revealed an invasive mammary carcinoma in the contralateral breast (false-negative mammography). Radiographic findings were calcifications and density/mass lesions in 27 and 8 cases, respectively. Of 27 cases presenting with Ca, 10 showed colocalization of LN and Ca. In the eight cases presenting with density/mass, incidental microscopic microcalcifications colocalized to LN were found in two cases. When present, histologic Ca was associated with LN in 12 of 29 cases studied (41%). Of the 21 patients with immediate or subsequent excision, five (24%) were found to have an associated invasive mammary carcinoma (two on immediate excision and three after short-term follow-up of up to 3 years). The bilaterality of cancer risk was expected; however, the number of invasive carcinomas was not. That the invasive carcinomas detected at follow-up were small implies that they might have been present (but occult) at initial presentation. We conclude that lobular carcinoma in situ detected on core biopsy is potentially a significant marker for concurrent and near-term breast pathology requiring complete intensive multidisciplinary clinical follow-up with specific individualization of patient care.