Left ventricular (LV) performance was assessed by echocardiography in 19 premature infants with severe respiratory distress syndrome. Measurements of LV size and function were made from digitized M-mode echocardiographic data in 14 babies (group 1) before and during treatment with mechanical ventilation and positive end-expiratory pressure. During ventilation, maximum LV dimension decreased (p = .001) as did peak filling rate (p = .01). LV shortening fraction decreased slightly (p = .05). There were marked reductions in calculated stroke volume (SV) (p = .001) and cardiac output (p = .0001) but systemic BP was unchanged, presumably due to peripheral vasoconstriction. The effect of simultaneous volume loading was studied in five other babies (group 2) who were ventilated under similar conditions. Blood transfusion with packed cells (10 mg/kg) prevented the fall in LV filling rate, while LV dimensions (max, p = .01; min, p = .02), SV (p = .05), cardiac output (p = .05), and systolic BP (p = .05) increased. This indicates that a low preload was responsible for the decreases observed in group 1.