Leiomyosarcoma of the breast is rarely encountered in fine-needle aspiration (FNA) cytologic material. We report a case of primary leiomyosarcoma of the breast in a 52-yr-old female. Aspiration cytology showed large, dissociated round to spindle cells with abundant vacuolated cytoplasm, pleomorphic nuclei, prominent nucleoli, and occasional intranuclear cytoplasmic invaginations. Mitotic figures, osteoclast-like giant cells, and stromal fragments were identified. A diagnosis of malignant neoplasm representing either a sarcoma, a poorly differentiated carcinoma, or a metaplastic carcinoma was made. The patient underwent a wide excision of the lesion after negative work-up. Histologic examination and immunohistochemical studies established the diagnosis of leiomyosarcoma. This case is presented here because we feel that, although FNA cytology with eventual ancillary studies is a valuable diagnostic tool to evaluate any breast mass, malignant spindle cell neoplasms of the breast still represent a diagnostic challenge for the cytopathologist. Recognition of all cytologic features of leiomyosarcoma may help to formulate a correct diagnosis. Diagn. Cytopathol. 2003;29:172–178. © 2003 Wiley-Liss, Inc.