Managing the continuous growth of modern-day IT developments and its transformation consequences in manufacturing and service organizations is arduous. It burdens organizations and supply chain management. For decades, supply chain managers have outsourced their non-specialized IT activities to third-parties. Competition compels organizations to look for ever more effective services from their suppliers. Solely outsourcing IT activities is no longer sufficient. Monitoring and optimization has become decisive. Lean, a successful philosophy to pursue continuous improvement, is extending its contributions from manufacturing and services to IT processes. This paper explores the success factors for the implementation of Lean in IT outsourcing relationships. It aids managers in designing a dashboard to optimize and monitor the implementation of Lean in IT outsourcing relationships. The causal-relationships among success factors in the context of a system, based on multiple case studies, is decoded through a causal-loop diagram and sensitivity analysis inspired by the system dynamics methodology.