Heparin resistance (HR) is occasionally encountered in cardiac surgery. The objective of this study is to identify possible factors predisposing patients to heparin resistance. Four hundred patients scheduled for elective cardiac surgery requiring the use of cardiopulmonary bypass (CPB) were enrolled in this study. Data collection included: demographic data, types of preoperative anti-coagulant/antiplatelet therapy, preoperative haematological and biochemical laboratory results, medical condition, baseline activated clotting time (ACT) and infusion volume prior to heparinisation. Patients who require 5 mg/kg or more to achieve ACT greater than 400 seconds before CPB were considered to be heparin resistant. All data was tested with a preliminary univariate analysis and factors associated with HR ( p value <0.05) were entered into a multivariate logistic regression analysis. Factors associated with HR with a p value <0.05 are considered predictors for HR. Thirty-two (8.0%) of the 400 patients fulfilled the criteria for HR. Preoperative heparin ( p = 0.025), low molecular weight heparin ( p = 0.049), plateletcount ≥ 300,000/ml ( p = 0.022) and albumin plasma concentration ≤ 35 g/dl 0, = 0.044) are significantly associated with HR. HR patients also tend to require more volume replacement prior to CPB to maintain haemodynamic stability p=0.007).