Thirteen patients with significant hemorrhage, severe thrombocytopenia, and megaloblastic bone marrows are described. Unusual features of this problem included its acute onset, frequent absence of the typical peripheral blood changes of megaloblastic anemia, normal serum B12 levels, and serum folates which were often not clearly abnormal. Most patients were critically ill and common clinical features included reduced dietary intake, renal failure, renal dialysis, the postoperative state, and sepsis. These clinical features, the laboratory findings, and a platelet increase in most patients after folate therapy lead to the conclusion that this problem is probably due to acute folic acid deficiency. Possible explanations for the atypical laboratory findings include the acuteness of onset, recent blood transfusion therapy, and impaired folate utilization. This problem may be relatively common. Because of its potential clinical importance, rapid onset, and attendant diagnostic difficulties, prophylactic folic acid is recommended in the clinical setting described.