Introduction: Capillary malformation-arteriovenous malformation (CM-AVM; MIM#608354) is caused by germline RASA1 and EPHB4 alterations. RASA1 intralesional second hits have also been reported. Constitutional mosaicism, defined as the presence of a mosaic variant in all cell types of an individual, is detected in clinical practice as mosaic variants in multiple tested samples from one individual or as mosaic variants in blood samples in a disorder affecting another cell/tissue types. Here we report RASA1 constitutional mosaicism in CM-AVM. Subjects and methods: A custom high-throughput sequencing panel was used to search for RASA1 pathogenic variants in blood samples from two unrelated patients with a clinical diagnosis of CM-AVM. An affected tissue sample from one of the patients was also analyzed. Results: Both patients showed different nonsense RASA1 variants in mosaic in blood samples and in the corresponding affected tissue sample from one of the patients. The mosaicism ranged between 7% and 21,5%. Conclusions: We report for the first time the presence of RASA1 constitutional mosaicism in CM-AVM. Constitutional mosaicism has implications for accurate molecular diagnosis and recurrence risk, and helps to explain the great phenotypic variability in CM-AVM.