In a large series of full-term infants born in Havana to women in very low economic circumstances, hemorrhagic disease of the newborn occurred with a frequency of 1 in 300 babies (26 cases). Vitamin K was not administered to the mothers or, prophylactically, to the babies. In the affected infants hemorrhagic manifestations appeared in the first 24 hours in 9, in 24 to 48 hours in 10 and in 48 to 72 hours in 7. The most common symptoms were hematemesis, melena and epistaxis. Investigation of the coagulation mechanism disclosed deficiencies of plasma thromboplastin component (PTC), plasma thromboplastin antecedent (PTA), proconvertin and prothrombin. By means of the thromboplastin generation test, deficiency of an additional serum factor required for thromboplastin formation was suggested. Studies in paper IV of the series indicated that this factor was the recently identified Stuart-Prower factor. All abnormalities of coagulation responded rapidly to administration of vitamin K, with most aberrations corrected in 24 to 48 hours and complete normality established in 48 to 72 hours. The deficient factors responded in specific sequence, with prothrombin and PTC rising most rapidly, then proconvertin and finally Stuart-Prower factor.