Background: The aim of this prospective study was to examine how age and standard clinical approach with and without CGA results determine treatment decisions in older cancer patients (pts). Methods: This study,conducted in 2 Belgian university hospitals, included pts ≥ 70 years with a malignant tumor (breast, colorectal, ovarian, lung, prostate and hematological) if a new cancer therapy was considered. All pts underwent a uniform CGA. Results were communicated to the treating physician. After the treatment decision, an interview with the treating physician was performed, using a predefined questionnaire: 1/ What would be your oncological treatment proposal in case the pt was 55y without other comorbidity? 2/ Is this different from your treatment proposal for this pt according to age and standard clinical approach without CGA results? 3/ Is this different from your treatment proposal for this pt with CGA results? Results: From October 2009 till July 2011, 937 pts were included in the study. Median age was 76y (range 70-95) and 63.5% was female. A total of 902 (96.3%) questionnaires were completed and 56.2% of the physicians were aware of the CGA results at treatment decision. In 381 pts (42.2%; 95%CI 39.0-45.5) age and standard clinical approach led to a different treatment decision compared to younger pts without comorbidity. This influence was most prominent for chemotherapy decisions: 309 patients did not receive standard chemotherapy (reduced dose (13), less toxic regimen (163), less toxic regimen at reduced dose (5) or no chemotherapy (128)). When the physician was aware of the CGA, these results influenced their treatment in 6.7% (95%CI: 4.5-8.9), mostly concerning chemotherapy. In 8 pts CGA results encouraged the treating physician to choose a more aggressive chemotherapy regimen and in 11 pts CGA results led to a decision of palliative care. Conclusions: Based on this prospective trial, we conclude that physicians use adapted treatment regimens in older versus younger pts, only based on age and standard clinical approach. CGA results change the treatment decision in 6.7% and sometimes trigger the use of a more aggressive treatment.