The discrepancy between serum CEA levels and CEA tissue expression in patients with breast cancer is well known. Whereas immunohistochemistry shows positive CEA expression in 70-90%, the serum CEA levels are often within the normal range. We performed immunoscintigraphy and SPECT with a Tc-99m labelled anti-CEA monoclonal antibody (MAb BW 431/26) in 46 women with suspected breast cancer or recurrence. The results of anti-CEA immunoscintigraphy, mammography, serum CEA levels and immunohistochemistry were evaluated according to the histology of the tumor. Histology verified breast cancer or recurrence (pT1 [n = 7], pT2 [n = 17], pT3 [n = 3], pT4 [n = 3]) in 30 out of 46 patients; benign breast disease such as fibrocystic disease, fibroadenoma, fatty necrosis or chronic mastitis was responsible for suspicious mammographic findings in 16 patients. Immuno-SPECT showed 25 true-positive, 5 false-negative, 11 true-negative and 5 false-positive findings (sensitivity 83%, specificity 69%). Anti-CEA immuno-SPECT of 2 patients with bone metastasis showed all lesions previously detected by bone scintigraphy to be CEA-expressing metastases. In contrast, serum CEA levels were slightly elevated in only 5 out of 30 patients with histologically verified breast cancer (sensitivity 17%). The results of immuno-histochemistry were surprising; tissue CEA expression could be demonstrated in only 5 patients with breast cancer. According to our experiences with this Tc-99m labelled anti-CEA MAb, immuno-SPECT is a suitable additional method for the diagnosis of breast cancer and especially of recurrence. Pre-operative serum CEA levels give no support for the differentiation between benign and malignant breast tumors.