The distinction between lobular and ductal lesions of the breast is important in several circumstances. Diagnostic reproducibility of lobular versus ductal lesions, based on histology alone, is less than optimal. The proper distinction between atypical lobular hyperplasia, lobular carcinoma in situ and low-grade ductal carcinoma in situ is critical for patient management. Patients who have a core biopsy of invasive lobular carcinoma often have preoperative magnetic resonance imaging to prepare the surgeon for proper margin attainment. E-cadherin, a negative membrane marker for lobular neoplasia, is useful in the distinction of lobular versus ductal neoplasia, but as a negative marker, can be difficult to interpret in particularly challenging cases. In this study, we surveyed primary and metastatic ductal lesions (62) and lobular lesions (64) of the breast to determine if P120 catenin is useful in the diagnostic distinction between lobular and ductal neoplasia. Primary breast ductal and lobular preneoplastic and neoplastic lesions were immunostained with E-cadherin and P120ctn and independently classified as ductal or lobular lesions. In addition, a wide array of carcinomas of different types were surveyed with P120ctn in tissue microarrays to ascertain whether the cytoplasmic P120ctn immunostaining pattern observed in lobular neoplasia was unique. Accurate categorization of ductal versus lobular neoplasia in the breast with P120ctn immunostaining was effective in all cases. Separation of low-grade ductal carcinoma in situ from lobular neoplasia was efficient. Diagnostically, P120ctn was particularly useful in identifying early lesions of lobular neoplasia. Of the other tumors that may morphologically mimic lobular carcinoma, only the diffusely infiltrating variants of rectal and gastric carcinomas showed diffuse cytoplasmic P120ctn immunostaining. Caution should be exercised when examining tumors in metastatic sites with P120ctn, with the incorporation of an appropriate panel of immunostains.