Congenital vascular malformations belongs to the group of rare diseases and treatment takes place in specialized centers. The HECOVAN is such specialized center. Treatment options for these vascular malformations are limited and consist of surgical extirpation to intralesional immunotherapy-sclerosis or both. Unfortunately this is not enough to help the patients. Persisting are the complaints of pain, invalidation and mutilating effect due to the vascular malformation making a normal functional life almost impossible. Recently promising results have been published regarding treatment with Sirolimus in this patient group reducing pain significantly and improving quality of life. Based on these case reports, we have treated in the meanwhile nine patients with amazing results. However, to justify treatment with Sirolimus in a larger patient group, it is necessary first to perform a study to determine true efficacy (pain reduction) but also to gain insight in possible adverse events that can occur when Sirolimus is used as a single agent. As vascular malformations are rare, it is difficult to obtain funding and find enough patients. Fortunately, Pfizer will support the present study with Sirolimus reducing the costs for performing such study significantly. In addition, all large centers in the Netherlands, treating vascular malformations, will participate making it possible to include enough patients. In addition, Pfizer was asked to provide Sirolimus on the basis of compassionate use for all patients with a positive reaction after the performance of the study if complaints of pain would return. Furthermore, Pfizer wants to support financially the database necessary for registration of adverse events during treatment and long term follow up. Especially for children, no data regarding long term consequences of sirolimus, are known and therefore it is essential to have such database. However, additional funding is necessary for the actual execution of the study.