Histologic examination using serial sections of a mastectomy specimen from an ILC patient with LCIS and unusual lobular proliferation showed the following: 1) Cancer cells and their infiltration patterns varied and were not typical of ILC, so that this case was categorized as ILC partially possessing some histologic features of IDC. 2) Unusual lesions of proliferation, which were initially regarded as ALH, consisted of both pure ALH and ILC invading the ALH and normal lobules. For the correct diagnosis of ALH in biopsy specimens, therefore, we must bear in mind the (possible) concomitant presence of ILC. 3) The histologic criteria for ALH and ILC must be defined more clearly.