A man was admitted with abdominal pain. Treatment for acute diverticulitis was instituted with intravenous antibiotics and oral limitation. Imaging demonstrated a complex inflammatory mass. Prothrombin time (PT), activated partial thromboplastin time (APTT) and fibrinogen were within normal limits. However, repeat preoperative clotting studies demonstrated a severe unexpected coagulopathy to have developed since admission that could have caused fatal intraoperative exsanguination. Direct assays showed severe, isolated deficiency of vitamin K dependent clotting factors, and mixing studies normalised both the PT and APTT, ruling out a coagulation inhibitor. The coagulopathy responded to intravenous vitamin K administration. Dietary insufficiency underlies vitamin K deficiency in the presence of normal biliary and enteral function. A significant coagulopathy can result with additional eradication of intestinal microflora. Hypoprothombinaemia is recognised as a consequence of protracted treatment with broad spectrum antibiotics, and vigilance is required for those at risk. The development of such a rapid and unexpected coagulopathy posed a complex preoperative management issue delaying operative intervention; although avoided by fortuitous preoperative screening, it could have caused significant intraoperative bleeding. The remarkably specific lack of vitamin K dependent clotting factors strongly suggested a vitamin K deficiency and administration of coumarins was ruled out.