Prognostic factors of Low Output Syndrome (LOS) requiring operative circulatory support by intra-aortic balloon-pumping (IABP) counterpulsation, were analysed in a population of 841 patients who underwent heart valve replacement surgery between June 1977 and May 1985. The incidence of IABP circulatory support was 6.8%. Mean survival time of patients who needed IABP was 2.35 years +/- 0.425 vs 6.30 years +/- 0.103 for patients who didn't have this complication. A multivariate analysis using the logistic model was done to pinpoint factors predictive of IABP support. The prognostic factors were pre-operative functional class, presence of prior valve replacement, presence of concomitant surgery and presence of endocarditic etiology. The subgroup of 58 patients undergoing balloon counterpulsation was analysed for factors predictive of survival using a multivariate analysis of the Cox' model. Presence of aortic or mitral regurgitation was found to be the independent risk factor of mortality. Using a combination of prognostic factors, we pinpointed groups of patients at high risk of needing post-operative balloon pump counterpulsation support. The necessity of an alternative procedure for the mechanical support of the failing circulation is underlined.