Background: Low-molecular-weight heparins (LMWHs) are increasingly used as anticoagulant during haemodialysis. The aim of this study is to establish the efficiency and duration of anticoagulation with dalteparin and nadroparin administration in patients treated with nocturnal haemodialysis. Methods: All patients were treated with nocturnal in-centre haemodialysis, 3-4 times a week. Anticoagulation was obtained with dalteparin (n = 15) or nadroparin (n = 10). Anti-factor- Xa activity was measured during a midweek dialysis session at t = 0, 4 and 8 hours. Results: The LMWH dose necessary to prevent extracorporeal circuit clotting was higher for dalteparin than for nadroparin. In the dalteparin group, anti-Xa activity was almost negligible at the start of dialysis whereas most patients on nadroparin still had anti-Xa activity at the start of dialysis (0.08 (IQR 0.05-0.11) IU/ml), reflecting the effect of previous LMWH administration. After eight hours of dialysis, median anti-factor-Xa activity was 0.49 (IQR 0.22-0.57) after dalteparin and 0.69 (IQR 0.55- .83) after nadroparin (p = 0.01). When a target range of 0.2-0.6 IU/ml was applied, the present dosing method led to over-anticoagulation in more than half of the patients. Conclusion: Administration of two doses of LMWH is an effective method of anticoagulation in nocturnal, eight-hour haemodialysis. With two doses of dalteparin, a larger proportion of patients reached but did not exceed target levels of anticoagulation, compared with two doses of nadroparin. Nadroparin caused prolonged anti-Xa activity with measurable anticoagulation up to the next dialysis session. The measurement of anti-Xa activity is advocated for dose assessment of LMWH, when LMWH is used as anticoagulant during nocturnal haemodialysis.