Lymphangiomatosis is a rare and fatal congenital lymphatic malformation. Because the natural course of the disease affects multiple body systems, the management can be challenging. This article presents a novel approach to the treatment of diffuse lymphangiomatosis using sirolimus. The reported case involves a 4-month-old male with a known lymphatic malformation who presented to the emergency department with respiratory difficulties. Sirolimus was successful at significantly reducing our patient's mass at a relatively low target level of 5 to 10 μg/L. The use of sirolimus for the treatment of lymphangiomatosis should be studied further in the setting of a formal trial.