Although gastrointestinal stromal tumor(GIST) occurs generally in the digestive tract,omental GIST is very rare.We report the first case of an adult greater omental GIST with a new platelet-derived growth factor receptor α gene(PDGFRA)-mutation with hemoperitoneum.A 43-year-old man was admitted to our hospital complaining of acute abdominal pain.Abdominal contrast-enhanced computed tomography revealed a huge mass in the right abdominal cavity,and a large accumulation of fluid in the pelvic cavity,suggesting hemoperitoneum.We diagnosed the rupture as an intra-abdominal tumor,and an emergency tumorectomy was performed with resection of the greater omentum.This tumor was located in the distal right side ofthe greater omentum,and showed no continuity with the gastric wall.The tumor occurred primarily in the greater omentum.The resected tumor was about 19 cm × 12 cm × 14 cm in diameter,and weighed 1529 g.Histologically,the tumor was composed of epithelioidshaped cells with high cellularity,and was positive for CD117 and CD34,and negative for S-100,α-smooth muscle actin.The mitosis was 6/50 under high power field.This case showed exon 18 mutation of PDGFRA with 846(Asp to Glu) substitution,848(Asn to Lys) substitution.This is the first report of this PDGFRA mutation in omental GIST,and this might play an important role in the tumorigenesis of this case.Based on these findings,the tumor was diagnosed as high risk GIST primarily occurring in the greater omentum.The patient was treated with imatinib at a dose of 400 mg/d as adjuvant chemotherapy,and has been followed up for 24 mo with no evidence of recurrence. Although the gastrointestinal stromal tumor (GIST) occurs generally in the digestive tract, the omental GIST is very rare. We report the first case of an adult greater omental GIST with a new platelet-derived growth factor receptor α gene (PDGFRA) -mutation with hemoperitoneum. A 43-year-old man was admitted to our hospital complaining of acute abdominal pain. Abdominal contrast-enhanced computed tomography revealed a huge mass in the right abdominal cavity, and a large accumulation of fluid in the pelvic cavity, suggesting hemoperitoneum. We diagnosed the rupture as an intra-abdominal tumor, and an emergency tumorectomy was performed with resection of the greater omentum. This tumor was located in the distal right side of the greater omentum, and showed no continuity with the gastric wall. greater omentum.The resected tumor was about 19 cm × 12 cm × 14 cm in diameter, and weighed 1529 g.Histologically, the tumor was composed of epithelioidshaped cells with high cellulari ty, and was positive for CD117 and CD34, and negative for S-100, α-smooth muscle actin. The mitosis was 6/50 under high power field. This case showed exon 18 mutation of PDGFRA with 846 (Asp to Glu) substitution , 848 (Asn to Lys) substitution. This is the first report of this PDGFRA mutation in omental GIST, and this might play an important role in the tumorigenesis of this case. Based on these findings, the tumor was diagnosed as high risk GIST [ occurring in the greater omentum. The patient was treated with imatinib at a dose of 400 mg / d as adjuvant chemotherapy, and has been followed up for 24 mo with no evidence of recurrence.