Two previously healthy children, ages 8 months and 35 months, developed spontaneous inhibitors to Factor IX. Brief illnesses of presumed viral origin preceeded hemorrhagic symptoms in both children. Cutaneous ecchymoses and traumatic soft tissue hemorrhage were the predominant clinical features in each case. Bleeding symptoms persisted for less than 3 days and laboratory evidence of Factor IX inhibition resolved within 3 weeks. One child required treatment with fresh frozen plasma and packed red blood cell transfusions. The other child received corticosteroid therapy. Given the transient nature of acquired Factor IX inhibitors in the nonhemophilic child, a conservative approach toward therapy is recommended unless life-threatening complications supervene.