Summary. Background: Vascular malformations (VMFs) are errors in the embryonic development of vessels and are always present at birth, but not necessarily visible or symptomatic. VMFs grow or dilate proportionally with the child and sometimes expand suddenly. They often get symptomatic at a later moment in life, mostly during childhood or adolescence. Objective: The objective is to elucidate the natural historyofVMFsinchildrenandyoungadultsandtofigure out how they are influenced in order to predict the course of the condition, to inform patients, parents and physicians about what awaits them and how they could possibly avoid progression or at least, be prepared to handle it. Methods: 51 patients displaying different types of vascular malformations at the Vascular Anomalies Center at the Medical University Vienna were included in this study. They were interviewed by questionnaire about the course of their disease, symptoms and possible triggers. Results: The patient collective was composed of 25 male and 26 female patients between 4 and 66 years of age. 37 had a slow-flow malformation (with capillary, venous or lymphatic components), 14 had a fast-flow malformation. Most of the malformations (64%) were localized on the extremities. 51% of them had their onset atbirth,37%inthefirst20yearsoflife,only12%laterthan 20 years of age. Symptoms at onset were swelling (75%), pain (45%), functional impairment (43%), skin discoloration (39%) and bleeding (6%). Some patients also experienced local infection at a later moment. 62% of the VMFs grew proportionally with age, 38% of the patients experienced a sudden progression of the lesion. Patients were classified into 4 growth patterns: 31% in 1a (onset at birth, proportional growth), 20% in 1b (onset at birth, progression), 31% in 2a (onset later, proportional growth) and18%in2b(onsetlater,progression).Factorsrelatedto onset and progression were puberty (24%), surgery (8%), trauma (6%) and pregnancy (4%). Factors that worsened the symptoms temporarily were exercise (78%), trauma (41%), heat (25%), stress (18%), illness (18%), cold (16%), pregnancy (8%) and hormonal contraceptives (6%). Conclusions: Due to the small group of patients and the diversity of vascular malformations, not all results were statistically relevant. Nonetheless, an epidemiologic overview is given and a contribution towards elucidation of the natural course of vascular malformations is provided.