A coagulopathy due to vitamin K deficiency was discovered in 42 hospitalized patients, most of whom had been misdiagnosed as having disseminated intravascular coagulation. Factors contributing to vitamin deficiency included inadequate diet, malabsorption, failure of physicians to prescribe vitamin K supplements, antibiotic therapy, renal insufficiency, hepatic dysfunction, recent major surgery, and possibly pregnancy. Sixteen patients (34%) bled sufficiently to need red blood cell transfusions and ten patients (24%) ultimately died. Of 18 patients who also had thrombocytopenia, three did have disseminated intravascular coagulation. The deficiency, a contributor to morbidity and mortality, can be prevented by prophylactic administration of vitamin K to severely ill patients who are eating inadequately and receiving antibiotics.