Cathepsin D is a protease which is known to facilitate invasion and metastasis of breast carcinoma. Overexpression of cathepsin D is associated with poor clinical outcome and biologic aggressiveness of the breast cancer. We underwent immunocytochemical assay(ICA) for cathepsin D in fine needle aspiration cytology(FNAC) specimens from the breast carcinoma and benign breast diseases. In FNAC specimens cathepsin D was expressed in 21(42.9%) out of 49 cases of invasive ductal carcinoma, whereas negative result was observed in all 15 cases of benign breast diseases including 7 fibroadenomas, 6 fibrocystic diseases, and 2 benign ductal hyperplasias. Among the 11 FNAC specimens from ductal carcinoma in situ(DCIS), cathepsin D was expressed in 3 cases(27.3%). In FNAC specimens immunocytochemistry for cathepsin D showed positive result in 24 out of 60 carcinomas(sensitivity, 40%) and negative result in 15 out of all 15 benign breast diseases(specificity, 100%). No significant correlation was noted between cathepsin D expression in FNAC specimen and clinicohistological characteristics of the breast carcinoma, such as hormone receptors and cell differentiation. In conclusion, ICA of cathepsin D in FNAC specimens thought to be a good adjunct to differentiate malignancy from benign breast diseases.