Primary breast lesions diagnosed by fine needle aspiration cytology, confirmed by histologic examination were analyzed by morphometry to evaluate the difference between benign and malignant lesions, and the methods obtaining the sample. Four size factors and 5 form factors were evaluated in 22 fibroadenomas and 20 carcinomas by image analyzer(Zeiss Ibas 2000) using the H-E stained slides. Nuclear size was significantly larger in the carcinoma cells than fibroadenoma cells both in the cytology and biopsy specimens, but the form factors were not significantly different. Both fibroadenoma and carcinoma cells were significantly larger in cytologic smear than histologic section. The cells in the cytology were more regular and round than those in histology, but not statistically significant. Fibroadenomas having cellular proliferation and atypism exhibited larger size and more irregular nuclei than non-proliferative fibroadenoma, but not statistically significant. Therefore nuclear morphometric analysis can be a helpful method to diagnose the questionable breast lesions and is a method appropriate for use as a quality control procedure in the fine needle aspiration cytology.