BACKGROUND The Frank-Starling law describes the relation between left ventricular volume and function. However, only a few studies have described the relation between left atrial volume (LAV) and function. OBJECTIVE To describe an LA Frank-Starling law by studying changes in LAV measured by real-time, three-dimensional echocardiography (RT3DE). METHODS LAV was calculated by RT3DE in 70 patients at end-systole (LAV(max)), end-diastole (LAV(min)) and pre-atrial contraction (LAV(pre-A)). According to LAV(max), patients were classified into three groups: LAV(max) <50 ml (group I), LAV(max) 50-70 ml (group II) and LAV(max) >70 ml (group III). Calculated indices of LA pump function were active atrial stroke volume (SV), defined as LAV(pre-A) - LAV(min), and active atrial emptying fraction (EF), defined as active atrial SV/LAV(pre-A) x100% RESULTS Active atrial SV was significantly higher in group II than in group I (mean (SD) 19.0 (9.2) vs 8.2 (4.9) ml, p<0.0001), in group III it was non-significantly lower than in group II (16.7 (12.5) vs 19.0 (9.2) ml). Active atrial SV correlated well with LAV(pre-A) (r = 0.56, p<0.001), but decreased with larger LAV(pre-A). Active atrial EF tended to be higher in group II than in group I (43.1 (18.2) vs 33.2 (17.5), p<0.10), in group III it was significantly lower than in group II (26.2 (18.5) vs 43.1 (18.2), p<0.01). CONCLUSIONS A Frank-Starling mechanism in the left atrium could be described by RT3DE, shown by an increase in LA contractility in response to an increase in LA preload up to a point, beyond which LA contractility decreased.