Summary Since low molecular weight heparins (LMWH) are partly eliminated by renal excretion, their pharmacodynamic profile may be modified in very elderly patients with age-related renal impairment. The aim of this prospective study was to determine whether tinzaparin (Innohep ® ) 175 anti-Xa IU/kg administered subcutaneously once daily over 10 days does accumulate in hospital patients greater than 70 years of age. Plasma anti-Xa and anti-IIa amidolytic levels and APTT were determined prior to the first injection (day 0), and then, at peak level i.e. 5 h after the second injection (day 2) and subsequently on days 5, 7 and 10. Thirty consecutive inpatients (6 men, 24 women) requiring LMWHs at a curative dose for acute thromboembolic disease were included. Patients’ characteristics (mean ± SD) were: age 87.0 ± 5.9 years (range 71–96), body weight 62.7 ± 14.6 kg (range 38-90) and creatinine clearance 40.6 ± 15.3 mL/min (range 20–72). The mean actual dose of tinzaparin delivered was 174.8 anti-Xa IU/kg. Since no patient had an anti-Xa activity above 1.5 IU/mL, the dose of tinzaparin remained fixed over 10 days. Anti-Xa and anti-IIa peak levels measured on day 2 were 0.66 ± 0.20 IU/mL (range 0.26–1.04) and 0.33 ± 0.10 IU/mL (range 0.18–0.55), respectively. Ex vivo anti-Xa/anti-IIa ratios were close to 2.1. APTT ratios (patient/control) were strongly correlated with anti-IIa activity (p <0.01). There was no progressive increase of the anti-Xa and anti-IIa activities after repeated administration of tinzaparin over the 10 day treatment period. No correlation was found between anti-Xa and anti-IIa activities and age, weight, or creatinine clearance. No major bleeding occurred during the study and only one minor haematoma at the injection site was reported. No thromboembolic complication or death occurred. Tinzaparin may thus be administered safely at a treatment dose (175 anti-Xa IU/kg) in older patients with age-related renal impairment. Neither dose adjustment, nor serial anti-Xa activity monitoring seems to be required in patients with creatinine clearance above 20 mL/min during the first ten day treatment.