Often patients are not weighed in hospital. Failure to weigh patients prescribed renally excreted drugs may correlate to adverse drug events. We carried out a cross-sectional study of patients prescribed common renally excreted drugs (heparin, enoxaparin and gentamicin), admitted to two wards at Royal North Shore Hospital, Sydney over 3 months. Of all patients surveyed, 28% (22/78) in the orthopaedic ward and 22% (27/124) in the medical ward were weighed. Among those prescribed therapeutic doses of the study drugs, 25% (3/12) in the orthopaedic ward and 27% (7/26) in the medical ward were weighed. Patients prescribed therapeutic anticoagulation who were not weighed experienced more haemorrhagic complications than patients who were weighed (P = 0.03). Patients prescribed renally excreted drugs in hospital are frequently not weighed. This is associated with reduced medication safety.