Fibrosarcomas are rare malignant soft-tissue tumors occurring mostly in infants younger than 1 year of age. Fibrosarcomas can ulcerate and cause various complications, which could threaten a fetus in utero or a child in the early neonatal period. We report a unique case of congenital infantile fibrosarcoma of the lower leg, its treatment and pathology. The large expansive and destructive lesion was not appreciated on routine prenatal ultrasound exams at 20 and 33 weeks gestation. The newborn required immediate emergency surgical intervention after delivery to prevent death by hemorrhagic shock. Initial debulking of the tumor was performed and hemostasis was attained on the day of birth. The child was resuscitated and definitive treatment of the leg was deferred until a pathologic diagnosis was obtained. Given the extent of the fibrosarcoma, the lower leg was not salvageable and the patient received a through-the-knee amputation in the neonatal period. The patient is free of disease at 2 years of age.