Background: Serum thioredoxin-1 (TRX-1) may be a useful biomarker for early diagnosis of breast cancer. Subject and Methods: Thioredoxin -1 and cancer antigen15-3 levels were determined using Enzyme-Linked Immunosorbent Assay (ELISA) in 141 women: 55 had primary breast cancer, 35 with benign breast tumor and 51 were apparently healthy controls matched for age, waist circumference and body mass index. Diagnosis involved all women with breast mass who referred to the Histopathology Department, Duhok Specialized laboratory Center during the study period. Fine needle aspirate (FNA) cytology test was carried out first and then the findings were later on compared with histological results to confirm the diagnosis. Breast cancer women were further grouped into subtypes based on immunohistochemistry. The aim of this study was to assess the diagnostic capacity of thioredoxin-1 for primary breast cancer.Results: Significantly higher Thioredoxine -1 and cancer antigen15-3 levels (p=0.001 and p=0.011 respectively) were found in primary breast cancer group compared to benign breast tumor group and healthy women. In breast cancer group, positive test was observed 2 times more likely than in benign breast tumor group as well in healthy women, with a sensitivity of 94.5%, specificity 72.1% and Receiver Operating Characteristics Curve ROC 0.872. Totally different and less significant values were observed for cancer antigen15-3, sensitivity 16.4%, specificity 93.0% and Receiver Operating Characteristics Curve ROC 0.352.Conclusions: Elevated thioredoxin-1 level in conjunction with a high sensitivity and specificity may be strongly associated with primary breast cancer in women; this may indicate the needs for determining serum thioredoxin-1 as a screening test for early diagnosis of primary breast cancer.