Introduction: The usefulness of morning routine chest X rays in under debate, some studies support their use, while on the other hand, some find marginal benefit. Objectives: To determine the diagnostic and therapeutic efficacy of morning routine chest X rays in mechanically ventilated patients and the identification of subgroups of patients in whom said X rays may be of greater usefulness. Methodology: Prospective, cross sectional, in patients under mechanical ventilation for minimum 48 hours, with age among 1 month to 18 years old, data recollected from December 2010 to November 2011. Contingency tables were done and later X2 Pearson function and Fischer´s exact test calculating the value of P, odds ratio, with confidence interval in the 95% for qualitative variables. For the data analysis we used programs Statgraphics and SPSS, version 15. Results: 53 patients and 536 chest X rays were evaluated. The findings were mostly minor. Only 10. 3% of the cases belong to major findings, mainly tube malposition and new infectious infiltrates. It was not possible to establish subgroups of patients but the data suggest greater benefit from X rays in patient with very young and low weight and those mechanically ventilated. Conclusions: The usefulness of routine films is marginal. Are needs more studies to establish the usefulness in patient’s subgroups. Key words: chest radiography, chest X ray, radiology, intensive care unit, critical care unit