Asthma is a widespread disease, affecting more than 300 million individuals. The treatment in children is based upon an administration of a pressurised metered-dose inhaler added with a spacer. The efficiency of drug delivery to the patient is strongly affected by the transient airflow pattern inside the spacer device. This paper presents a computational fluid dynamics (CFD) analysis of airflow inside a commercially available spacer device with wide application. This study, carried out in Fluent™, was the basis of an optimisation procedure developed to improve the geometry of the spacer and develop a more efficient product. The results show that an appropriate control of the boundary layer development, by changing the spacer shape, reduces the length of the recirculation zones and improves the flow. It can be concluded that CFD is a powerful technique that can be successfully applied to optimise the geometry of such medical devices.