The purpose of this study was to determine the contributions of pulmonary mechanics, gas exchange, and bemodynamic function to prediction of success or failure with early weaning from mechanical ventilation after cardiac surgery. More than 40 physiologic measurements were determined on 162 patients at two time points in the early postoperative period: during full-support mechanical ventilation and during a trial of spontaneous ventilation. Discriminant analysis was used to analyze the differences between the success group (n = 134) and the failure group (n = 28) and to predict group membership of individual cases. A moderately strong relationship (canonical correlation =0.733) was found between the groups and the discriminating variables. The discriminant function contained variables from all three dimensions of weaning criteria, with dimensions of hemodynamic function and pulmonary mechanics having greater import. The predictor set had a senstivity of 98, specificity of .71, positive predictive value of. 94, and negative predictive value of .87. Routine bedside measurements had a predictive performance equal or superior to variables previously studied.