Gastrointestinal stromal tumor (GIST) typically occurs in the gastrointestinal (GI) tract, and expresses KIT protein that is associated with KIT or platelet-derived growth factor receptor-α (PDGFRA) gene mutation. Extragastrointestinal stromal tumors (EGISTs) are a minor subset of GIST that occurs in the soft tissue outside the GI tract, and in very rare cases, these tumors can be KIT negative. We examined the clinicopathologic and molecular characteristics of 10 cases of KIT-negative EGIST by using immunohistochemical staining and gene mutation analysis. The tumors occurred in the omentum (n=5), mesentery (n=2), retroperitoneum (n=1), pelvic cavity (n=1), and not otherwise specified regions of the abdominal cavity (n=1). They ranged from 4 to 33 cm (median, 15 cm) in maximum diameter with relatively low mitotic counts (median, 3.5 per 50 high-power fields). Morphologically, most cases were of epithelioid cell (n=9) or mixed epithelioid and spindle cell (n=1) type, accompanied by variable amounts of myxoid stroma. By immunohistochemical staining, the tumors were positive for CD34 (80%), protein kinase C (PKC) θ (90%), and discovered on GIST-1 (DOG1) (90%), but were negative for KIT (0%). The majority of the examined cases (7 of 9 cases; 78%) had PDGFRA mutations in exon 12 (n=1) or exon 18 (n=6). One case (11%) had a mutation in KIT exon 11, and the remaining 1 had no mutation in either KIT or PDGFRA. Distant metastasis and local recurrence occurred in 1 (10%) and 2 (20%) patients, respectively, and adverse outcome was correlated with larger (>10 cm) tumor size and high mitotic counts (>5/50 high-power fields). Therefore, KIT-negative EGISTs can be characterized by preferential omental origin, epithelioid cell type, low mitotic activity, and mutation of the PDGFRA gene, and these features are similar to those of KIT-negative gastric GISTs. As KIT-negative EGISTs should be considered to be a potential abdominal soft tissue neoplasm, immunohistochemical staining panel and molecular analysis are necessary not only to confirm the diagnosis but also to determine the therapeutic strategy.