Sub-aortic (SA) aneurysms are a rare entity of variable aetiology. We report the first case of a SA aneurysm assessed using cardiac magnetic resonance imaging (MRI). A 33-year-old female with human immunodeficiency virus and on highly active antiretroviral treatment presented with syncope and dyspnoea. Clinical examination suggested moderate to severe aortic regurgitation (AR) confirmed by transthoracic and transoesophageal echocardiograms. However, echocardiography was suboptimal in defining the precise mechanism and severity of AR. A cardiac MRI was done to elucidate the aetiology, severity and mechanism of regurgitation. It confirmed the presence of a SA aneurysm below the left coronary cusp and its retraction, resulting in an eccentric AR jet. An assessment of moderate AR, based on regurgitant volume, was made. Furthermore, the anatomical relationships of the aneurysm were clearly defined. Cardiac MRI allowed comprehensive assessment of this SA aneurysm.