The prevalence and causes of erythrocytosis after liver transplantation have never been studied, even though this condition is known to predispose patients to thrombosis leading to graft failure or death. Erythrocytosis after orthotopic liver transplantation (OLT) can be defined as an increase in the red cell mass >125% in patients without a pre-OLT history of this condition. The study population was composed of 96 patients: 33 had undergone transplantation for a hepatitis B virus (HBV) infection (18 had a hepatitis D virus coinfection), 43 had undergone transplantation for a hepatitis C virus infection, 9 had undergone transplantation for alcohol abuse, and 11 had undergone transplantation for other causes [autoimmune liver disease (6), Wilson's syndrome (1), or cryptogenetic liver cirrhosis (4)]. Idiopathic erythrocytosis was reported in 11 male patients with a history of HBV infection. Patients with the diagnosis of erythrocytosis underwent phlebotomy every 3 weeks until the hematocrit level reached 45%, and this was repeated if the level exceeded 49%, so no patient presented with cardiovascular accidents during the follow-up. In conclusion, a history of HBV infection, male sex, and hepatitis B immune globulin therapy are all possible cofactors for an increased risk of erythrocytosis in OLT patients. Liver Transpl 19:420–424, 2013. © 2013 AASLD.