Remarkable leukocytosis with obscure abdominal pain during pregnancy is clinically challenging for obstetricians. A 31-year-old pregnant woman developed persistently elevated white blood cell (approximately 30 000/µL) and C-reactive protein (3.0 mg/dL) with occasional moderate abdominal pain. At 29 gestational weeks, she underwent emergency cesarean section due to suspected abruptio placentae. Hemoperitoneum was observed with extensive hemorrhagic nodules on the peritoneal and omental surfaces. White blood cells rose 87 200/µL, and C-reactive protein peaked at 44.9 mg/dL after surgery. Pathologically, biopsies showed deciduosis, and decidual cells on the omental surface showed immunohistochemical staining for granulocyte colony-stimulating factor (G-CSF). Serum G-CSF concentration was 339 pg/mL at 28 weeks, and that of ascites was 312 000 pg/mL at cesarean section. G-CSF-producing deciduosis can induce leukocytosis as well as abdominal pain during pregnancy and postpartum.