Background : Little interest has been paid to expiratory muscle strength, and the impact of expiratory muscle weakness on critical outcomes is not known. Very few studies assessed the relationship between maximal expiratory pressure (MEP) and critical outcomes. The aim of this study was to investigate the relationship between MEP and critical outcomes. Methods : This work was a secondary analysis of a prospective, observational study of adult patients who required mechanical ventilation for ≥ 24 h in an 18-bed ICU. MEP was assessed before extubation after a successful, spontaneous breathing trial. The relationships between MEP and extubation failure, and short-term (30 days) mortality, were investigated. Univariate logistic regressions were computed to investigate the relationship between MEP values and critical outcomes. Two multivariate analyses, with and without maximal inspiratory pressure (MIP), both adjusted using principal component analysis, were undertaken. Unadjusted and adjusted ROC curves were computed to compare the respective ability of MEP, MIP and the combination of both measures to discriminate patients with and without extubation failure or premature death. Results : One hundred and twenty-four patients were included. Median age was 66 years (IQR 18) and median mechanical ventilation duration was 7 days (IQR 6). Extubation failure rate was 15% (18/124 patients) and the rate for 30-day mortality was 11% (14/124 patient). Higher MEP values were significantly associated with a lower risk of extubation failure in the univariate analysis [OR 0.96 95% CI (0.93–0.98)], but not with short-term mortality. MEP was independently linked with extubation failure when MIP was not included in the multivariate model, but not when it was included, despite limited collinearity between these variables. This study was not able to differentiate the respective abilities of MEP, MIP, and their combination to discriminate patients with extubation failure or premature death (adjusted AUC for the combination of MEP and MIP: 0.825 and 0.650 for extubation failure and premature death, respectively). Conclusions : MEP is related to extubation failure. But, the results did not support its use as a substitute for MIP, since the relationship between MEP and critical outcomes was no longer significant when MIP was included. The use of MIP and MEP measurements combined did not reach higher discriminative capacities for critical outcomes that MEP or MIP alone.