Fine-needle aspiration cytology and flow cytometry of intracystic papillary carcinoma of breast To define criteria for cytologic diagnosis of intracystic papillary carcinoma (ICPC), the authors retrospectively reviewed the fine-needle aspiration (FNA) cytologic specimens of eight cases of histologically proven ICPC of breast. The patients were five black and three white women, 56-87 years of age. The FNA specimen was cyst fluid in four cases (bloody in three, clear in one). All the aspirates showed cellular smears with small and large clusters of cells with papillary and/or cribriform configurations and numerous single epithelial cells. The cells were cuboidal to columnar with minimal atypia. ICPC was suggested by FNA in each case, and all the patients underwent surgical excision or mastectomy. Flow cytometry, performed on fresh FNA specimen in one case and on paraffin-embedded surgical tissue in all eight cases, showed seven tumors to be aneuploid and one to be diploid. The authors contend that ICPCs of breast have distinct cytomorphologic features that can be recognized by FNA. Because ICPC may present in cyst fluid, either bloody or clear, all breast cyst fluids from postmenopausal women should be examined cytologically. Flow cytometric demonstration of an aneuploid population may assist in confirming malignancy in this lesion. 