e11028 Background: Recent studies have reported a greater mean breast density associated with estrogen receptor (ER)-positive disease than with ER-negative tumors, but controversial results have been published. The purpose of our study was to compare the difference in the distribution of breast cancer subtypes detected either by mammography screening or tumors diagnosed with clinically palpable masses in our centre. Methods: Between June 2008 and December 2011 157 patients with age between 40 and 70 years-old were diagnosed of early stage breast cancer. Tumors were classified in three major breast cancer subtypes by immunohistochemistry: TN (ie, ER, PR and Her2 negative), Her2-positive (ie, Her2 positive; ER and PR may be positive or negative), and ER-positive/Her2 negative (ie, ER positive, Her2 negative, PR may be positive or negative) types. Tumor characteristics and type of diagnosis (mammography screening or palpable masses) were obtained by retrospective chart review. Associations between categorical variables were evaluated with the X2 test. Results: Seventy patients (44.6%) were diagnosed of breast cancer with screening mammography, whereas 85 (54.1%) cases were detected by clinical suspected masses. Distribution according to molecular subtype defined by IHC were as follows: 123 (78.3%) ER-positive/Her2-negative tumors, 15 (9.6%) TN tumors, and 19 (12.1%) Her2-positive tumors. Less Her2-positive and TN tumors were significantly diagnosed with mammography screening (p=0.039 and p=0,020, respectively). No statistical differences were showed between ER-positive/Her2-negative tumors diagnosed with mammography screening and clinical palpable masses (p=0.78). Conclusions: The results presented in our study suggest that biologically aggressive subtypes of breast cancer are less frequently diagnosed with mammography screening in comparison with ER-positive/Her-2 negative tumors. Nevertheless, given the limited number of patients included and the bias related to retrospective studies these results must be interpreted with caution.