520 Background: BC diagnosed in women 35 years of age or younger is associated with aggressive clinical-pathological characteristics and poorer disease free survival. Our understanding of the biological processes associated with early onset BC is limited. Methods: We compiled clinical and gene expression profiling data from 4,831 early-stage BC patients across 31 datasets with a median follow-up of 6.6 years. Tumors were assigned to a molecular subtype and expression scores were computed for several published oncogenic pathway signatures. The chi-square and Mann-Whitney tests were used for between group comparisons. Distant metastasis free survival (DMFS) was estimated using the Kaplan-Meier method and compared by the log-rank test. Hazard ratios (HRs) were calculated using a Cox-proportional hazards model. Results: The pattern of molecular subtypes differed according to age at diagnosis (age ≤ 35: triple negative [TN] 39%, HER2 20%, LumA 16%, LumB 25%; age > 35: TN 19%, HER2 16%, LumA 30%, LumB 35%; p < 0.001). Young age was associated with an increased risk of distant metastasis (10-year DMFS 52.8% age ≤ 35 versus 69.1% age > 35, p = 0.002) that remained statistically significant after adjustment for known prognostic factors (HR 1.54, 95%CI 1.04-2.28, p = 0.031). This increased risk was most apparent in the TN (HR 2.05, 95%CI 1.10-3.90, p = 0.025) and LumB (HR 2.52, 95%CI 1.15-5.51, p = 0.021) subtypes. Compared with older patients (age>65), there was higher expression of proliferation, PTEN loss, and immune response and lower expression of stromal gene expression signatures (all p < 0.001) in young TN patients. In the LumB subset, young patients had increased expression of PI3K activated genes (p < 0.001). Histopathological validation of these genomic findings is ongoing. Conclusions: TN BC is more common in women ≤ 35 years of age. Young age is associated with a poor outcome from breast cancer, particularly for the TN and LumB subtypes. Increased expression of proliferation, immune response, and PTEN loss gene signatures in TN and PI3K activation in LumB suggests possible targets for biological investigation and future clinical trials in this high-risk population. No significant financial relationships to disclose.