UNLABELLED: The breast cancer is one of the most frequent in the world, with an incidence that has been raising despite the many different prevention programs. In the early 1980s, two groups of investigators report a new tumor marker and called it cancer antigen 15-3 (CA 15-3). CA 15-3 in the diagnostic and follow up of patients with breast cancer has a sensitivity ranging from 75-76.9% and the specificity ranging from 85.5-93%. METHODS: We review the files of 100 female patients in the Nuclear Medicine Department, which was done bone scan and CA 15-3 from January to December 2000. All patients were in stage III and IV. For the bone scan every patient received 20 mCi of Medroxi-Di Phosphonate (MDP). The CA 15-3 was quantify in the radioimmunoanalysis (RIA) laboratory. The CA 15-3 reference protocol is equal or less than 30 U/mL. RESULTS: The mean patient's age was 59.39 years. The mean value from CA 15-3 for the patients with the absence of metastatic disease is 16.18 U/mL and 164.02 U/mL in the presence of metastatic disease mL (p < 0.00001). In our research the high level was 35 U/mL (percentil 95). The sensitivity founded was 82% and the specificity was 87%. High CA 15-3 levels are present when the patient has the tumor and get down to normal when the tumorectomy of the mastectomy was done; in the absence of metastatic disease. CONCLUSIONS: Skeletal scintigraphy is reserved for patients with clinical stage III and IV disease. Image those patients who have skeletal pain or high levels of CA 15-3 although an early stage disease. In patients with skeletal metastases and chemotherapy protocol there is a significative diminish of the value of the tumor marker in comparison with the previous one. In patients with metastatic disease and normal CA 15-3, the tumor marker will increase gradually. CA 15-3 can be use as a simple method that reflects the presence of bone metastases in association with bone scan.