Muscular hamartoma (MH) is a rare tumor-like lesion of the breast, and only 22 cases have been reported in the literature. We report an additional case of MH of the breast, giving a new immunocytochemistry-based insight into the histogenesis of the smooth muscle component. The expression of CD34 antigen in smooth muscle cells suggested that MH originates from stromal cells via leiomyomatous metaplastic changes. This hypothesis was supported by the evidence that stromal cells in the normal parenchyma nearby the hamartoma were positive for CD34 antigen and, focally, also positive for smooth muscle markers (alpha-smooth muscle actin and desmin). A complete list of differential diagnoses and a review of the literature on the topic are provided. Although the use of the term "muscular hamartoma" could be considered inappropriate because of the absence of smooth muscle tissue in the normal breast, the authors underline that this designation can be easily accepted if the histogenesis of the heterologous component is regarded as a result of a stromal cell metaplastic change.