One hundred and eighty-eight cases obtained for aspiration biopsy cytology (ABC) of invasive breast cancers were investigated to elucidate the relationoship between histological types and morphological featuers, such as the prevalence of small-sized carcinoma cells, mean longitudinal diameter values of the cytoplasm and nucleus of the small-sized carcinoma cells, and the morphological features of these cells, and their cytological backgrounds.The cases showing predominantly small-sized carcinoma cells accounted for 45. 7% of all cases examined (86/188). The prevalences of small-sized carcinoma cells among histological types were as follows; papillotublar carcinom (18/44, 40.9%), solidtublar carcinoma (12/51, 23.5%), scirrhous caricinoma (45/81, 55.8%) and invasive lobular carcinoma (11/12, 91.7%).<BR>The mean longitudinal diameter values of the cytoplasm depended on histological types and ranged from 11 to 14 pm, whereas those of the uncleus were consistently at approximately 7μm.<BR>Morphological features of these cases usually allowed delineation of their histological types.<BR>It was noteworhy that we recognized the “solitary scattered pattern” in all histological types, the cases of which were presumed to have solid-tubular carcinoma based on morphological features.<BR>We performed cytomorphological studies of the “solitary scattered pattern”, and recognized characteristics of the cells or the background, allowing identification of the histological types.In general, small-sized cancer cells were recognized in scirrhous and invasive lobular carcinomas. More importantly, these cells were seen in both papillotubular and solidtubular carcinomas. Thus, cases showing small-sized cells, should be evaluated for histological type on the basis of morphological features and the “solitary scattered pattern” should be evaluted precisely in terms of the individual cytoplasm, nuclei and background of each small-sized cell.