The purpose of this study was to evaluate left atrium dimension during acute volume loading. Low molecular weight dextran solution (1,000 ml) was continuously infused in 10 subjects without cardiac disease. Measurements of pulmonary arterial, capillary wedge, right atrial and systemic blood pressures, cardiac output, heart rate and M-mode left atrial maximum diameter were made in basal conditions and after 250, 500, 750 and 1,000 ml of dextran. In 4 subjects a two-dimensional echocardiogram was also obtained in the short axis and four-chamber views. Dextran produced the following changes (mean values ± standard deviation) in haemodynamic variables: right atrial and wedge pressures increased from 3.2 ± 1.55 to 12.3 ± 3.08 mm Hg (p < 0.001) and from 6.9 ± 1.45 to 20.6 ± 3.44 mm Hg (p < 0.001), respectively. Stroke volume increased from 70.5 ± 31.6 to 86.6 ± 24.6 ml/beat (p < 0.05). Heart rate increased from 81.0 ± 15.8 to 95.0 ± 12.3 beats/min (p < 0.05). Left atrial maximum diameter showed a slight incrase after 500 ml of dextran (from 39.5 ± 4.1 to 43.6 ± 5.0 mm, p < 0.05). It did not further increase by continuing the infusion up to 1,000 ml despite the marked increase in wedge pressure. The results show that left atrial size is partly independent of haemodynamic changes and that the compliance of the left atrium is relatively low. This finding is consistent with the common clinical observation that the left atrium is only slightly enlarged in acute mitral insufficiency.