Objective: Intracardiac thrombosis is a life-threatening complication of extreme prematurity. We describe the use of tissue plasminogen activator to treat intracardiac thrombosis in extremely low-birth-weight preterm infants. Design: Case series, literature review, and practice guideline for recombinant tissue plasminogen activator treatment of intracardiac thrombosis in extremely low-birth-weight preterm infants. Setting: Neonatal intensive care. Patients: Four extremely low-birth-weight preterm infants coincidentally diagnosed with intracardiac thrombosis during neonatal intensive care. Interventions: Recombinant tissue plasminogen activator in a starting dose of 20 μg/kg/hr, increasing to 200-400 μg/kg/hr, infused for 2-6 days. Measurements and Main Results: Thrombolytic therapy with recombinant tissue plasminogen activator helped achieve rapid clot resolution in all infants and none had any major hemorrhagic complication associated with treatment. Conclusions: Tissue plasminogen activator may safely be used to treat intracardiac thrombosis in extremely low-birth-weight preterm infants. Close monitoring of therapy is imperative. Further data are required to confirm the safety of tissue plasminogen activator in preterm infants.