The effect of PEEP on cardiac performance was evaluated in 21 patients with left ventricular (LV) dysfunction. Twenty-three data sets were divided into three groups according to pulmonary arterial wedge pressure (PAWP). In three of four group A data sets (PAWP = 12 mm Hg), cardiac output (CO) decreased when PEEP was added. In four of six group B data sets (PAWP = 14–18 mm Hg) and in 12 of 13 group C data sets (PAWP ≥ 19 mm Hg), CO increased with addition of PEEP. In group C, the mean increase in CO was 500 ml/min, and the mean level of best PEEP was 3.9 cm H2O. When PAWP exceeded 18 mm Hg, PEEP was safe and in many instances augmented CO.