Guideline-based decision support systems have been developed to influence the prescribing behaviour of clinicians, but they have not yet shown to increase physician compliance with best practices in routine. OncoDoc is a non-automated system that allows flexibility in guideline interpretation to obtain best patient-specific recommendations at the point of care. OncoDoc is applied to breast cancer management. We have experimented the system at the Institut Gustave Roussy with a before-after study in which treatment decisions for breast cancer patients were measured before and after using the system in order to evaluate its impact upon physicians' prescribing behaviour. After 4 months, 127 decisions were recorded. Physicians compliance with OncoDoc was significantly improved (p < 10(-4) ) to reach 85.03% after using the system. Comparison of initial and final decisions showed that physicians modified their prescription in 31% of the cases. Clinical trial accrual rate increased of 50%, though not statistically significant because estimated on small figures.