Low molecular weight heparins (LMWHs) are commonly used in paediatric tertiary institutions for primary prophylaxis and treatment of thromboembolic events. Despite this widespread use, the therapeutic and prophylactic guidelines for LMWH therapy in children are extrapolated from adult guidelines. In fact, there is very little information regarding the pharmacokinetics, clinical effectiveness, adverse event profile and optimal dose schedule for LMWH therapy in children. This study was designed to determine whether paediatric-specific dosage requirements for LMWH are justified, by investigating the doses required to achieve target therapeutic ranges. Patients who were treated with enoxaparin between October 2003 and July 2007 were identified for inclusion in this study. One hundred forty patients had an anti-activated factor X assay result with a total of 55 (39%) patients achieving therapeutic levels 4–6 h post dose. Children younger than 1 year required the highest dose of enoxaparin/kg and highest number of dose changes to achieve the target therapeutic range. Major bleeding occurred in one patient, equating to 0·7%, with complete clot resolution recorded in 16 (11%) patients. This study demonstrated a 2–3-fold variation in individual dose requirements for LMWH in children ≤5 years of age, and further mandates the need for age-specific dosage requirements in children receiving enoxaparin.