Enoxaparin is a low molecular weight heparin (LMWH) that has been shown to be effective in deep vein thrombosis, pulmonary embolism, and unstable angina. Because renal function plays an important role in the clearance of LMWH, the authors sought to investigate the effect of renal function on enoxaparin. This prospective multiple-dose study evaluated 18 patients with varying degrees of renal function initiated on enoxaparin 1 mg/kg subcutaneously every 12 hours. Peak blood levels of anti-Xa concentrations were obtained 4 +/- 0.5 hours postdose after receiving at least three doses of enoxaparin. The median antifactor Xa levels were higher in patients with creatinine clearance (CLCr) < or = 30 mL/min compared to CLCr > or = 31 mL/min (1.34 IU/mL vs. 0.91 IU/mL, respectively, p < 0.05). A linear correlation was established between creatinine clearance and anti-Xa concentrations (p < 0.0005). On the basis of the data, the authors believe that a dose adjustment is necessary in patients receiving repeated doses of enoxaparin with CLCr < or = 30 mL/min.