A combined vascular malformation involves two or more types of vessel abnormalities. Any of the four types of lesions (capillary, venous, lymphatic, or arteriovenous) may be combined (illustration 1). Combined malformations are typically accompanied by overgrowth of soft tissues (skin, fat and muscle) and bones. However, patients may also have other skeletal abnormalities and undergrowth of the affected side. Most combined malformations involve limbs, but they can involve any area of the body. While the precise cause of these lesions is unknown, they begin early in embryonic life and are thought to be caused by an error in the formation of multiple types of blood vessel channels in a particular anatomical area. Their occurrence is unrelated to drugs or medications that may have been taken during pregnancy or to environmental exposures that may have occurred at that time.