This study was designed to determine if the activated partial thromboplastin time (aPTT) present at the end of a diagnostic cardiac catheterization was correlated with specific patient or procedure characteristics. No relationship between time from heparin bolus and the aPTT was found. Age, weight and body surface area were found to influence the aPTT at catheterization completion (p < 0.05). Patient sex, height, smoking status, hematocrit, and type and volume of contrast used were not correlated with aPTT at the end of catheterization. This suggests possible usefulness of the age and weight of patients to guide the dose of heparin used at cardiac catheterization.