Abstract: This study was carried out to determine the sensitivity and specificity of serum CA 15-3 as a marker in detecting and monitoring treatment in, breast cancer patients. One hundred and ten patients comprising 35 known breast cancer patients, 75 suspected cases and 20 controls entered the study. Blood samples were taken before and after treatment from the 35 known cases as well as the 75 suspected cases from which biopsy specimens were also taken. Serum CA 15-3 was measured by BioCheck CA 15-3 Enzyme Immunoassay. There was a significant difference between the concentration of serum CA 15-3 of the 35 known breast cancer patients before and after treatment (p<0.05). Out of the 75 suspected cases, 46 had breast cancer and 29 had benign breast disease (histologically proven). There was a strong positive correlation between the level of serum CA 15-3 and the histopathology results of the biopsies (r = 0.518). The mean serum CA 15-3 concentration of the 46 patients (80.6+70.2 U mL-1) was significantly higher (p<0.05) than that of the 29 patients with benign breast disease (12.0+9.0). The sensitivity and specificity of the serum CA 15-3 in detecting breast cancer was 76.1 and 100%, respectively at a cut-off of 35 U mL-1. Serum CA 15-3 was found to have a value in the early detection and monitoring of treatment of breast cancer in Ghana.