A 56-year-old man was diagnosed to have a huge gastric cancer extending from the lesser curvature of the stomach to the pancreas with multiple hepatic and peritoneal metastases. Two days after completing chemotherapy with cisplatin plus high dose leucovorin and fluorouracil, drastic necrotising tumour lysis led to gastric perforation and septic shock most likely due to bacterial peritonitis. The image of tumour lysis looked like an emphysematous pancreatitis. Afterwards, immunohistochemical study of the tumour specimen confirmed moderate positivity of dihydropyrimidine dehydrogenase and absence of Bcl-2 expression. The incomplete expression of dihydropyrimidine dehydrogenase and total deficiency of Bcl-2 are considered to be the main underlying causes of such extraordinary chemosensitivity and so severe a tumour lysis phenomenon. Pre-emptive intensive survey of possible biomarkers of chemosensitivity is thus highly recommended upon treating a massive gastric cancer.