Abstract The goal of sclerotherapy is to obliterate abnormal channels by damaging the endothelium, thereby resulting in subsequent inflammation and fibrosis. Venous malformations (VMs) are caused by abnormal development of the vein wall, with thinning and asymmetric disruption of the smooth muscle layer of the vein in association with endothelial cell abnormalities. This results in progressive, often asymmetric, dilation of the affected channels. Associated absence or insufficiency of valves in the conducting veins contributes to swelling. Affected channels become progressively enlarged, and the resulting stagnation of blood causes thrombosis, swelling, and pain. Most VMs undergo a continuous cycle of spontaneous thrombosis and thrombolysis. Calcification of thrombi results in formation of phleboliths. Symptoms and signs include blue or purple cutaneous lesions, swelling with dependency or effort, pain, deformity, and consumption coagulopathy. Pulmonary embolism can occur, especially when the conducting venous channels are malformed.