Occult carcinoma cells in peripheral blood of breast cancer (BC) patients is generally associated with poor disease prognosis. Reverse transcriptase-polymerase chain reaction (RT-PCR) is a sensitive method for revealing rare circulating cancer cells. The target mRNA must be carefully chosen, as it must be expressed only by malignant cells. In this study, we developed a nested RT-PCR assay for mammaglobin (hMAM) and assessed both its specificity and its sensitivity in the detection of cancer cells in peripheral blood of BC patients. hMAM mRNA was detected by RT-PCR in 156/165 (95%) of fresh BC tissues analyzed. All samples from 66 healthy blood donors and 151 patients with benign breast disease were hMAM negative as assessed by nested RT-PCR. In contrast, hMAM was detected in 16/137 (12%) of peripheral blood samples deriving from BC patients: 0/9 in stage 0, 1/50 (2%) in stage I, 3/33 (9%) in stage II, 1/18 (5%) in stage III and 11/27 (41%) in stage IV. Using nested RT-PCR, we were able to amplify hMAM transcript of one tumour cell/10(6) normal cells. Our data demonstrate that hMAM mRNA detection by RT-PCR is a specific assay potentially suitable for identification of occult cancer cells in peripheral blood of BC patients.