BACKGROUND: Preoperative low-molecular-weight heparin (LMWH) is often used when warfarin therapy is interrupted for surgery. OBJECTIVE: To determine the preoperative anticoagulant activity of LMWH following a standardized "bridging" regimen. DESIGN: Prospective cohort study. SETTING: Single university hospital. PATIENTS: Consecutive patients who had warfarin therapy interrupted before an invasive procedure. INTERVENTION: Enoxaparin, 1 mg/kg of body weight, twice daily. The last dose was administered the evening before surgery. MEASUREMENTS: Blood anti-factor Xa heparin levels measured shortly before surgery. RESULTS: Preoperative anti-Xa heparin levels were obtained in 80 patients at an average of 14 hours after the last dose of enoxaparin was administered. The average anti-Xa heparin level was 0.6 U/mL. The anti-Xa heparin level, measured shortly before surgery, was 0.5 U/mL or greater in 54 (68%) patients and 1.0 U/mL or greater in 13 (16%) patients. A shorter interval since the last dose (P < 0.001) and a higher body mass index (P = 0.001) were associated with higher preoperative anti-Xa heparin levels. LIMITATIONS: The small sample size limits accurate estimates of the frequency of the clinical outcomes. A single regimen of LMWH was evaluated. CONCLUSIONS: Anti-Xa heparin levels often remain high at the time of surgery if a last dose of a twice-daily regimen of LMWH is given the evening before surgery.