Abstract BACKGROUND AND OBJECTIVES: Breast cancer in very young patients (< 35 years) (BCVY) is an uncommon disease and when it occurs it usually has aggressive biological characteristics. Whether this effect is due to an overrepresentation of aggressive breast cancer subtypes in younger patients or not remains an issue of controversial. The objective of this study was to identify potential differences in the molecular and clinical features of breast carcinomas from patients < = 35 years versus a cohort of older counterparts previously matched by breast cancer subtype. METHODS: We performed a retrospective analysis of a prospectively maintained database that included 424 patients diagnosed with an invasive breast carcinoma from 1995 to 2012 at Hospital Clinico of Valencia. We selected 89 patients separated in two groups, the study group with very young women ≤ 35 years old and with no BRCA mutation or unknown and a second group with women older than 50 years. Data related to clinical and pathological features from both groups such as tumor size, nodal status, histological grade, Ki 67 labeling oestrogen and progesterone receptor and HER2 overexpression were obtained from medical records and we used the statistic model of chi-squared to compare the two groups. RESULTS: Of the 89 patients, 43 patients ≤ 35 year were included in the study group (median age 31.4 years, standard deviation (SD): 3.82) and 46 patients >50 years were included in the control group (median age 66.4 years, SD: 10.00). Ductal carcinoma was the most common histological subtype in both groups (88.4% of BCVY and 78.3% of the old woman). The majority of tumors were ER and PR positive in both groups, but younger women had a higher proportion of HER2 positive tumors, although the result was not significant. By subtype 57.4% of BCVY presented an immunohistochemical luminal subtype, compared to 71.7% in older patients. Triple negative and HER2 profiles were 11.6% and 34.8% in youngest versus 15.2% and 13.04% in older women respectively. No statistically significant differences were found in terms of breast cancer subtype (p = 0.279)). However, BCVY had a higher pathological grade (56% younger patients had grade IIII vs 26% in oldest, p = 0.002), higher proportion of Ki67 >30% (32.5% of youngest versus 17.4% in older women, p = 0.008), larger tumors (16.3% of BCVY had size tumor > 5 cm, versus 2.2% in controls, p<0.0001) and more frequent nodal involvement (39.5% in young women vs 28.2% in oldest, p = 0.0264). CONCLUSIONS: Patients diagnosed with breast cancer at ≤ 35 years present more aggressive tumors compared to older patients. These data suggest that BCVY is a distinct entity, further studies to confirm these findings are needed. Citation Information: Cancer Res 2013;73(24 Suppl): Abstract nr P5-13-14.