Production grids are complex and highly variable systems whose behavior is not well understood and difficult to anticipate. The goal of this study is to estimate the impact of the variability of those infrastructures on the performance of workflow-based applications. A probabilistic model of workflows execution time is proposed and evaluated. Results show that the variability of the EGEE grid infrastructure impacts the execution time of a particular medical image analysis application by a factor 2. The model gives interesting insights on the grid behavior for different application parallelization modes. I. PERFORMANCE ANALYSIS ON PRODUCTION GRIDS In many scientific areas, applications with stringent requirements for high performance computing, large data sets analysis and complex computation flows have emerged. Pushed by these new computational challenges very large scale production grids infrastructures have been deployed world-wide. Such widely distributed systems have been operating...