Lipomatous hypertrophy of the interatrial septum is characterised by an accumulation of fatty tissue in the interatrial septum. The authors report three cases, one presenting with sinus tachycardia and the other two being chance findings. Echocardiography associated with cardiac computerised tomography or magnetic resonance imaging usually confirms the diagnosis. In half the cases, supraventricular arrhythmias and suggestive P wave abnormalities are observed on the electrocardiogram. The diagnostic value of transoesophageal echocardiography is emphasised; it demonstrates the massive forms which may obstruct flow from the superior vena cava into the right atrium. The authors observe a discrepancy between the prevalence of this condition in autopsy series (about 1%) and the small number of cases described at echocardiography, suggesting that the diagnosis is probably missed.