Objectives: Low-molecular-weight heparins are widely used in clinical practice for the treatment or prophylaxis of venous thromboembolism (VTE). As these drugs are eliminated mainly by renal means, any renal function impairment may lead to higher plasma concentrations and increase the risk of bleeding. This study aims to evaluate whether in clinical practice there is an increase in the occurrence of bleeding in patients with renal insufficiency (RI) during treatment or prophylaxis with dalteparin, and to analyse the risk factors potentially influencing the appearance of such bleeding events., Methods: Patients were sampled from the Universitary Severo Ochoa Hospital, Leganés, Spain. This was a retrospective cohort study with a 1 year inclusion period, conducted at a Spanish university hospital with 400 beds, on patients undergoing treatment or prophylaxis for VTE with dalteparin for a minimum of 3 days. The main outcome measure was the number of patients who had bleeding events, independently of their severity, during dalteparin administration in patients with RI., Results: 367 patients were included in the study. Bleeding occurred in 17.9% of patients in the group with RI and in 7.3% of patients with normal renal function (NRF). Most haemorrhages in both cohorts were grade 2 on the WHO scale (64.7% in the RI group and 69.2% in the NRF group). Logistic regression analysis allowed the presence of RI (MDRD-4 (Modification of Diet in Renal Disease) <50 mL/min) to be identified as a risk factor., Conclusion: Patients with RI treated with dalteparin face a higher risk of bleeding than those with NRF, which seems to make it necessary to monitor and seek new dosage adjustments for these patients.Impact on practice statements: This study yields new data on dalteparin in RI, which has not been widely studied before., Competing Interests: Competing interests: None declared., (© European Association of Hospital Pharmacists 2022. No commercial re-use. See rights and permissions. Published by BMJ.)