In 70 patients with KIT D816V positive systemic mastocytosis (SM) including 36 patients with advanced SM (AdvSM), we correlated the extent of reported mucosal mast cell ([m]MC) infiltration of the upper and/or lower gastrointestinal tract (UGIT, n = 63; LGIT, n = 64; both, n = 57) with symptoms and markers of MC burden/subtype. This evidence concerns the gene KIT and systemic mastocytosis.