Low-grade solid in situ carcinomas of the breast are difficult to classify. The authors investigated 12 cases of in situ carcinomas with equivocal features and correlated their histologic attributes with those of the associated invasive carcinomas as well as with E-cadherin expression in both in situ and invasive disease. E-cadherin-positive in situ lesions were invariably associated with invasive carcinomas of the ductal type. In situ carcinomas that were E-cadherin negative were associated with invasive carcinomas of the lobular type in five of six cases. In all cases, the invasive carcinomas showed the same pattern of E-cadherin reactivity as the in situ lesions. Sharply defined cellular membranes, necrosis, and occasional microacini were seen in both E-cadherin-positive and negative in situ carcinomas, whereas intracytoplasmic lumina and a noncohesive appearance were seen only in E-cadherin-negative lesions.