Two cases of hypoprothrombinemia of unknown cause are presented. In each case there was a marked hemorrhagic diathesis which, in Case 1, led to death. In each there were additional findings (clinical or necropsy) which suggested that the prothrombin deficiency was only one manifestation of a generalized disease.

The pertinent literature is reviewed and a classification of the previously reported cases of "idiopathic hypoprothrombinemia" into "congenital" and "acquired" types is offered. Both of our cases fall into the "acquired" group. The treatment of the condition is discussed and it is pointed out that vitamin K1 oxide in large doses restored the prothrombin time to normal after the apparent failure of large doses of water soluble menadione. A rationale for this difference in response is suggested.

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