Abstract : Serum ferritin level is a routine check to monitor iron overload in thalassemia patients. Iron overload due to transfusion can lead organ damage, such as causing vascular damage which is at risk of reminding the incidence of thrombosis. Patients with thalassemia frequently have increased plasminogen activator inhibitor-1 (PAI-1) levels from the side effects of blood transfusion, such as vascular complications like inflammation, vasculitis, and thrombosis. There may be a relationship between serum ferritin level and serum PAI-1 level. To determine the relationship between serum ferritin level and serum PAI-1 level. An analytical cross-sectional study was carried out to thalassemia patients who receiving routine blood transfusions in Haji Adam Malik hospital, Medan from May to June 2021. Serum ferritin and PAI-1 levels examinations were carried out through blood sampling. Pearson and Spearman correlation tests were used for normal and abnormal distribution data analysis. Statistical test with p <0.05 was considered significant. The lowest ferritin serum level was 445.86 ng/mL and the highest level was 12836.44 ng/mL. The lowest PAI-1 serum level was 0.12 ng/mL and highest level was 10.76 ng/mL. There was no significant relationship between PAI-1 serum level and serum ferritin level (p=0.454). Age, transfusion length, and transfusion volume were significantly correlated with PAI-1 levels with positive correlation value. No significant relationship between serum ferritin level and the increased in serum PAI-1 level. Age, transfusion length, and transfusion volume were significantly correlated with serum PAI-1 level.