Vitamin K deficiency bleeding cases in Thailand from 1963 to 1995 were extensively studied. From 1963 to 1987 there were 499 reported cases from 10 papers including 102 cases of the authors' series. From March 1994 to April 1996, two subsequent nationwide surveys were conducted where questionnaires were sent to 714 and 732 hospitals located throughout Thailand. The responding rate was 58.2% and 67% respectively. 331 cases were found during 1988 to 1995. The total number was 830 cases of which 799 were idiopathic vitamin K deficiency in infancy (IVKDI) and 31 were secondary types. IVKDI was found exclusively breast-fed infants (92%) who did not receive vitamin K prophylaxis at birth (90%). Bleeding and pallor were the common features. The occurrence of intracranial hemorrhage was strikingly high (82%); the fatality rates was 24%. However, the fatality rate among patients receiving either 1 mg of vitamin K, intramuscularly, (17%) or 2 mg, orally, (18%) were lower than those not receiving vitamin K prophylaxis (36%). The incidence of IVKDI significantly declined to 4.2-7.8 per 100,000 births between 1988 to 1995 which was in reverse proportion to the coverage of vitamin K prophylaxis (r = -0.94, p < 0.05).