Background: Previous studies have described differences in breast cancer biological subtypes from various ethnic backgrounds such as women of African descent. Data on the heterogeneity of breast cancer biology among Asian ethnic groups is limited. Objectives: The aim of this study was to evaluate the frequency of the different immunohistochemical (IHC) subtypes of breast carcinoma and clinical outcomes among Chinese, Malay and Indian patients (pts) at a single Singaporean institution. Methods: Local pts of Chinese, Malay and Indian ethnicity seen at the National Cancer Centre Singapore (NCCS) and diagnosed with breast cancer from 2001 to 2004 were included in this retrospective study. Tumours were classified using IHC profiles as 1)"Luminal": (ER+ and/or PR+, HER2−), 2) HER2 overexpressing (HER2+ regardless of hormone receptor status) and 3) Triple Negative (TNBC: ER, PR and HER2−). Data was collected on pt and tumor characteristics including stage, grade, body mass index (BMI), treatment received including chemotherapy and endocrine therapy, as well as disease-free survival(DFS) and overall survival(OS). Statistical analysis was performed using PASW statistics version 18. Results: Out of the total of 2,061 pts in this study, 86.6% were Chinese, 8.7% were Malays and 4.7% were Indians. Among pts with fully defined receptor status(n=1,427), Chinese pts were more likely to have “luminal” breast cancer at 57% compared with 45% among Malays (p=0.005) and 50% among Indians (p=0.11, ns). Malay and Indian pts were more likely to have TNBC compared with Chinese pts (25.2% and 25% versus 16.6%, p=0.0082 and 0.027 respectively). The frequency of HER2−overexpressing breast cancer did not differ significantly among the Chinese(26.6%), Malay(30.1%) and Indian pts(25%). A greater proportion of Malay pts presented at more advanced stage of 3 or 4 than Chinese and Indian pts (45.6% vs 32.1% and 31.5%, p<0.001 and 0.01 respectively), and were diagnosed at an earlier age than Chinese and Indians (48.5y vs 52.3y and 52.2y, p<0.001 and 0.05 respectively). The frequency of grade 3 cancers was higher among Malays and Indians compared with Chinese (52.3% and 56.3% vs 42%, p=0.007 and 0.006 respectively), and had higher BMI compared to Chinese pts (26.4 and 26.2 vs 22.8, p<0.001 and <0.001 respectively). Malay pts also had lower 5-yr DFS at 59 % and 5-yr OS at 76% compared with 76%(p=<0.001) and 84%(p=0.04) in Chinese pts. Survival analyses on Indian pts were limited by the small numbers. Full multivariate analyses will be tabulated. Conclusions: There are significant differences in the frequency of various subtypes of breast cancer among the Chinese, Malays and Indians in Singapore. The disparities in survival outcomes between the Chinese and the Malays may be related to more aggressive tumor biology, more advanced stage at diagnosis and host factors such as higher BMI. The full impact of ethnicity on breast carcinogenesis is still unknown but is increasingly relevant with the development of therapies that target specific molecular pathways. Citation Information: Cancer Res 2011;71(24 Suppl):Abstract nr P2-14-07.