Children (birth to 21 years of age, inclusive) with first-episode venous thrombosis in association with a reversible clinical trigger (key exclusions: history of cancer; severe thrombophilia state disclosed) are enrolled and prescribed anticoagulation according to the clinical standard of care and American College of Physicians (Chest journal) 2012 recommendations. At the 6 week (post-diagnosis) follow-up visit, repeat radiologic imaging is performed to determine residual thrombus burden and its degree of occlusion. In addition, those subjects with antiphospholipid antibodies (APA) disclosed at enrollment will undergo repeat APA testing.