The relationship of tissue carcinoembryonic antigen (CEA) to clinicopathological factors and prognosis was investigated in 168 patients with invasive ductal carcinoma of the breast. Tissue CEA was determined by radioimmunoassay and a level of 5.1 ng/ml or more considered to be positive. Tissue CEA was positive in 31.5 per cent of the patients overall and, of the clinicopathological factors, tumor size and the presence or absence of lymph node involvement were not found to be correlated with tissue CEA. However, the tissue CEA positivity rate was significantly higher in patients who had four or more metastatic lymph nodes (p less than 0.01). Tissue CEA-positive patients showed earlier recurrence than CEA-negative patients (p less than 0.01) and had a poorer outcome (p less than 0.05). Thus, tissue CEA is considered useful as a prognostic index for primary breast cancer patients.