Coagulation and platelet function in 13 children with acute lymphoblastic leukemia were studied sequentially during a remission induction with L-asparaginase, prednisone, and vincristine. In the first weeks of therapy, which included four doses of L-asparaginase coagulation was characterized by significant decreases in plasma concentrations of plasminogen, antithrombin III alpha 2-macroglobulin, and fibrinogen. All measures gradually returned to normal after complication of L-asparaginase therapy. In the latter part of induction treatment, clotting times, especially partial Thromboplastin time, decreased significantly, while levels of factors V and VIII increased with recovery of platelet counts. At this time, 6 patients had an increased in vitro platelet aggregation response to adenosine diphosphate, and their partial thromboplastin times were significantly shorter than those of patients without increased aggregation. Concurrent abnormalities in coagulation and platelet function may account for the thrombotic complications that develop in some children receiving induction therapy with these agents.