Detection of non-palpable (T0) breast cancer by pathological nipple discharge is possible 3 years or more earlier than tumorous breast cancer. However, the definite diagnosis of T0 breast cancer has been considered to be very difficult because the standard diagnostic method such as exfoliative cytology and ductography were not totally reliable. In 1985 we first demonstrated the significance of CEA measurement in nipple discharge for diagnosis of T0 breast cancer. Since then CEA activity in nipple discharge was estimated in 60 patients with breast diseases by means of enzyme immunoassay using monoclonal anti CEA antibody. They include 17 with T0 breast cancer, 9 with borderline lesion, 20 with intraductal papilloma and 14 with fibrocystic diseases. When the cut off value of CEA concentration was set at 600 ng/ml, the sensitivity, specificity and accuracy were 76.5%, 100% and 92.2%, respectively. These levels were higher than those for mammography or cytology. In the past 7 years, 13 cases of T0 breast cancer were detected in our hospital. They accounted for 2.5% of total breast cancer cases. In conclusion, CEA measurement in nipple discharge is a useful method for the diagnosis of non-palpable breast cancer.