Spiradenocarcinoma is a very rare malignant tumor. In situ adenocarcinoma has recently been observed and defined by the preservation of a peripheral myoepithelial cell layer. The pathway for this phenomenon has been hypothesized to involve a sequence of adenomatous changes followed by atypical adenomatous changes, adenocarcinoma in situ, and invasive adenocarcinoma. However, there are no clearly defined morphological distinctions between atypical adenomatous changes and adenocarcinoma in situ. The authors present a case of spiradenocarcinoma in a preexisting spiradenoma in the left inguinal area of a 71-year-old woman. Adenomatous changes, atypical adenomatous changes, adenocarcinoma in situ, and invasive adenocarcinoma were found within the same lesion. The majority of the malignant component was an in situ adenocarcinoma. Although a loss of the myoepithelial cell layer and coalescing, irregular, glandular nests were present in several discrete foci, this case did not show infiltrative growth beyond the fibrous connective tissue. The authors speculate that this case represents a very early invasive spiradenocarcinoma lesion. We present this case to discuss the histological and/or immunohistochemical criteria of atypical adenomatous changes and to discuss the optimal treatment in cases with a disparity between the preservation of the whole architecture and the loss of the myoepithelial cell layer, which may be associated with favorable biological behavior.