The purposes of this study were to determine the frequency of vitamin K deficiency and to assess its effects on bleeding in patients with acute and intractable diarrhea. A total of 90 children with diarrhea and 30 healthy children (group C) were included in this study. Patients were divided into 2 groups, according to the duration of diarrhea. Complete blood count; prothrombin time (PT); activated prothrombin time (APTT); Factors II, VII, IX, and X; and protein C levels were studied in all patients. A total of 3 mg of vitamin K was administrated to patients with prolonged PT and/or APTT. Coagulation parameters were restudied 8 to 12 h after vitamin K was administered. Mean age, sex, weight, and breastfeeding percentage, as well as history of fever and vitamin K administration at birth, were similar in the 2 groups. The duration of antibiotic administration in group B (intractable diarrhea) was significantly longer than that in group A (acute diarrhea). Gastrointestinal (GI) bleeding was observed in 3 (4.9%) infants in group A and 6 (20.7%) in group B (P< .05). The duration of diarrhea was significantly longer in infants with GI bleeding. Intracranial bleeding occurred in 1 infant with intractable diarrhea. Prolonged PT levels were noted in groups A and B. Significant improvement in PT and APTT and an increase in coagulation factors were observed after vitamin K had been administered. Investigators in this study conclude that coagulation parameters can be improved by the administration of vitamin K to children with deranged PT and APTT and diarrheal illness.