The tumor marker CA 15-3 is one of the most import reliable for metastatic breast cancer monitoring. While it is generally assessed in serum of patients, blood sampling is an invasive method compared to saliva sampling which is simple and could be an alternative to blood according to many studies. The aim of this investigation was to assess the relationship between serum and salivary concentrations of the protein CA 15-3 in patients with breast cancer and healthy asymptomatic volunteers. A case-control study was conducted with 60 women: 29 breast cancer patients from the Maternity Hospital Souissi Rabat (Morocco) and 31 healthy asymptomatic women. The CA 15-3 concentrations in saliva and serum samples were assessed using an enzyme immune assay (EIA kits) and comparison between cases and controls was made by the Mann-Whitney test. The correlation between serum and saliva CA 15-3 concentration was tested using Pearson correlation. The comparison result of CA15-3 concentration in saliva and serum level in cases and controls was not statistically significant (p>0.05). However, the correlation between salivary and serum CA 15-3 concentration was positive and statistically significant (r=0.27, p=0.03). In conclusion, the positive correlation between salivary and serum expression found in our study suggests that saliva could be an alternative to blood sampling to help breast cancer monitoring.