This study investigates the means by which hospitals can improve the reliability of their medical supply value streams. Field research involving pharmacy supplies in eight Australasian public sector hospitals utilised a rigorous, multi-method data collection procedure based around the uncertainty circle model. Value stream maturity, performance inhibitors and pathways to effective improvement are explored. The high levels of value stream uncertainty detected are commonly the result of process immaturity and poor inter-functional integration. Chief among the common root causes is failure by the executive to acknowledge the strategic value of medical supplies management, which ultimately leads to higher materials cost and increased risk of a medical mishap. The study demonstrates how managers can use systems thinking and a context-free performance benchmark to identify effective interventions and potentially transferable best practices.