Abstract: This invention is directed to citrulline therapy, where citrulline is administered to achieve a newly recognized therapeutic window. The results of a completed clinical trial has elucidated a therapeutic window for L-citrulline. Detailed analysis of clinical outcome as a function of the dose actually received reveals that patients falling within the dosing window showed significant improved effect compared to patients receiving a dose either below or above the dosing window. Patients benefiting from this invention are patients in need of supplemental citrulline to ensure that adequate nitric oxide levels are produced by endothelial tissues.