Objectives: Clinical studies have shown low toxicity and a favorable safety profile for sirolimus in vascular anomalies. Here, we describe severe adverse events (SAEs) observed during “off-label use”. Methods: We performed a retrospective, multicenter chart review for SAEs during “off-label” sirolimus therapy for vascular anomalies and analyzed these cases by a pre-designed workflow. Results: We identified 17 SAEs in 14 patients diagnosed with generalized lymphatic anomaly (n=4), Gorham-Stout disease (n=2), central conducting lymphatic anomaly (n=1), lymphatic malformation (n=4), tufted angioma (n=1), kaposiform hemangioendothelioma (n=1), and venous malformation in a CLOVES syndrome (n=1). Three patients presented two SAEs. The age at initiation of sirolimus therapy was under the age of 2 years (5x), 2 to 6 years (5x) and older than 12 years (4x). SAEs occurred during the first 3 months of sirolimus therapy (7x), between 3 and 12 months (7x) and after one year of therapy (3x). The most frequent SAE was viral pneumonia (8x) resulting in death due to a metapneumovirus infection in a 3 months old and generalized adenovirus infection in a 28 months old child. Sirolimus blood level at the time of SAEs ranged between 2.7 and 21 ng/L. Five patients were on antibiotic prophylaxis during sirolimus therapy. Conclusions: Most SAEs are observed in the first year of sirolimus therapy; however, SAEs can also occur after a longer treatment period. SAEs are potentially life threatening, especially in early infancy. Presence risk factors, i.e. underlying vascular anomaly or immune status, may contribute to the risk of SAEs.