Unfractionated heparin (UFH) therapy is commonly used for the management of acute coronary syndrome (ACS), atrial fibrillation (AF) or atrial flutter (A-Flutter). Weight based heparin nomogram is widely used to determine the dosage of continuous intravenous infusion. Frequent monitoring is required due to the variable responses to its anticoagulant effects. We investigated factors that may influence the dose of UFH. We performed a retrospective cohort study of all patients who received continuous UFH for ACS, AF, or A-Flutter from January 2008 to December 2008 and reached maintenance dose of UFH within 48 hr. A total of 199 patients were included in this study: 42.7% were females and mean age was 66.6 ± 2.0 years. Univariate analysis revealed that age, gender, weight, serum albumin, and serum protein level were significantly associated with maintenance of UFH dose. After adjustment with multivariate analysis, age (R = -0.27, P ≤ 0.001), gender (R = -0.14, P ≤ 0.044), and weight (R = 0.54, P ≤ 0.001) remained to have significant association with maintenance UFH dose. In conclusion, we have identified not only weight but also age and gender as significant factors that may affect the maintenance of heparin dose in clinical setting.