Abstract Background Adjuvant! Online is a free web-based tool which predicts 10-year breast cancer outcomes and efficacy of adjuvant therapy in patients with breast cancer. As its prognostic performance has only been validated in high income Caucasian populations, the model was validated in a middle income Asian setting. Material and Methods: Within the University Malaya Hospital-Based Breast Cancer Registry, all 631 women receiving standard surgery for invasive non-metastatic breast cancer between 1993 and 2000 were identified. Calibration of Adjuvant! Online was evaluated by comparing the predicted 10-year overall survival with observed 10-year survival. Discrimination of the model was tested by receiver operating characteristic (ROC) analysis. Results: For the entire cohort, Adjuvant! Online predicted 10 year survival (70.3%) was significantly higher than the observed 10 years survival (63.6 %) by a difference of 6.7% (95%CI: 3.0%- 10.4%). The model was especially overoptimistic in women under 40 years and in women of Malay ethnicity, where survival was overestimated by approximately 20% (95%CI: 9.8- 29.8%) and 15% (95%CI: 5.3−24.5%) respectively. Adjuvant! Online performed fairly in terms of discrimination, with an area under ROC curve of 0.73 (95%CI: 0.69−0.77). Conclusion: Even though Adjuvant! Online is capable of discriminating between good and poor survivors, it systematically overestimates survival. These findings suggest that the model requires adaptation prior to use in Asian settings. Citation Information: Cancer Res 2011;71(24 Suppl):Abstract nr P2-12-20.