The objective of this study was to assess a weight-based heparin (WBH) nomogram (80-U/kg bolus, 18-U/kg-per-hour initial infusion) and determine its clinical performance and impact on resource utilization. All patients treated with heparin for venous thromboembolism or unstable angina during a 15-week study period were included in this retrospective, chart-review study. Three groups were identified: patients treated with WBH, patients whose regimen deviated from the weight-based nomogram (DEV), and matched historical controls (HCs). In patients receiving heparin for more than 24 hours, those treated with WBH achieved threshold activated partial thromboplastin time (aPTT) levels significantly faster than did HC or DEV patients. However, 42% of WBH-treated patients were found to have initial supratherapeutic responses. Logistic regression analysis identified age > or =67 years, prior warfarin therapy within 7 days of heparin, and high initial infusion rate as predictive of a supratherapeutic aPTT response; smoking was predictive of a subtherapeutic response. Bleeding events were not significantly different between groups. An infusion rate of 15 U/kg per hour was found to closely approximate our population's actual heparin infusion requirement. Resource utilization was significantly different between the WBH and HC groups in terms of nursing interventions at 48 to 72 hours. We concluded that WBH rapidly drives patients' aPTT response above the therapeutic threshold for heparin; however, prudent adjustment of the initial infusion rate is necessary to avoid a supratherapeutic aPTT response. Our data support a nomogram with an initial infusion rate of 15 U/kg per hour.