We include those patients who present with an isolated metastasis of axillary adenopathy in the occult primary breast cancer group. Presumably, the primary tumor is a primitive breast carcinoma, unsuspected until this moment and not clinically demonstrable by mammography or ultrasonography. When no method succeeds in confirming the primary breast lesion, the patients are usually treated assuming the existing of breast cancer. Several diagnostic methods have been used to find the primary breast lesion. Magnetic Resonance imaging (MRI), Positron Emission (PET) and Doppler sonography have been used in this way and several papers present the results reached with them. Our group incorporates detection and radioguided surgery with 99mTechnetium (99mTc) methoxyisobutil isonitrile into the study of these patients. We perform a planar scintimammography and SPECT (Single Photon Emission Computed Tomography) with 99mTc-MIBI. If the radioisotopic method shows a functional image compatible with a carcinoma, a gamma detecting probe is then used to locate the lesion and guide its surgical removal. In this paper, we present the application of the technique in 5 cases and describe the technique and its possibilities. Its advantages are explained in comparison with other methods. The dosimetric values found in the performance of the technique are reported. We consider that detection and radioguided surgery with 99mTc-MIBI in the diagnosis and treatment of occult breast cancer adds an effective tool and means progress in the approach to this disease.