The prevalence, natural history, and implications of reactive thrombocytosis after liver transplantation (LT) are unknown. Prospectively collected data from July 2000 to February 2006 were analyzed. Post-LT thrombocytosis was defined as a platelet count of ΟΎ450 Ο« 10 3 /L lasting for ΟΎ7 days and starting within 8 weeks of transplantation. In patients who survived ΟΎ8 weeks, graft and patient outcomes were compared with liver transplant recipients who survived ΟΎ8 weeks and did not develop any thrombocytosis. Post-LT thrombocytosis was seen in 92 (14.7%) of 627 patients. The median onset was on day 13 (range, days 1-44) and the peak platelet count was seen on day 17 (range, days 3-110). The median duration of thrombocytosis was 25 days (range, 7-1,253 days), with a median peak platelet count of 625 Ο« 10 3 /L (range, 472-1,381 Ο« 10 3 /L ). Seronegative fulminant hepatic failure was the indication for transplantation in 18% of patients with post-LT thrombocytosis compared with 3% of controls (P Ο½ 0.001). There was a lower proportion of patients transplanted for hepatitis C-related cirrhosis in the thrombocytosis group (10% vs. 18%, P Ο 0.04). The occurrence of hepatic arterial thrombosis was similar in the 2 groups (5% vs. 4%, P Ο NS). None of the 4 patients with platelet count higher than 1,000 Ο« 10 3 /L developed thrombotic complications. Post-LT thrombocytosis is more often associated with seronegative fulminant hepatic failure, and there is a negative association with hepatitis C-related cirrhosis. Post-LT thrombocytosis does not increase the risk of hepatic artery thrombosis, and patients without thrombotic complications should not be treated.