The evaluation of the diaphragm in critically ill patients is simple and noninvasive and has shown good repeatability. The aim of the study was to generate a predictive index for successful weaning (ULDIMex) from invasive mechanical ventilation (IMV) based on an ultrasonographic diaphragmatic assessment before performing a spontaneous breathing test. We recruited patients who required IMV and who were candidates for weaning from ventilation. The measurement of diaphragmatic excursion and time during inspiration and expiration was obtained with a 3- to 5-MHz probe in the M mode. Using the formula (a + b)c/2, the value for the ULDIMex was obtained to evaluate its impact on predicting the successful weaning of IMV, where a is the time during the inspiratory phase, b is the time during expiration, and c is the diaphragmatic excursion during the inspiratory phase, which corresponds to the highest point of the curve from the baseline. We recruited 114 patients, of whom 86 (76%) were successfully weaned from IMV. The patients who were successfully weaned from the IMV had a cutoff value greater than 4.06 cm/s for the ULDIMex index, with a sensitivity of 92.8% (95% confidence interval, 76.5-99.1), specificity of 63.9% (95% confidence interval, 52.9-74.0), positive predictive value of 45.6%, and negative predictive value of 96.5%. The ULDIMex index demonstrated a good level of discrimination for successful weaning prediction. Considering the excellent negative likelihood ratio of the ULDIMex index of greater than 4.06, this index may be considered before performing an spontaneous breathing test to identify critically ill adult patients who will extubate successfully.