Purpose To compare between lung ultrasound and chest X-ray in diagnosis of different lung pathologies in critically ill patients using lung CT as a gold standard. Methods Comparative prospective randomized single group observational study was conducted in the Critical Care unit (medical and surgical ICU). The study was conducted upon fifty patients (28 females &22 males) with a mean age of 58 years (SD ± 15.55; (19 - 82) presented to Kom Hamadah Hospital during a period of one year starting from July 2018 to July 2019, with acute dyspnea as the primary complaint or developed acute dyspnea and or tachypnea during their ICU stay were evaluated for the presence of pleural effusion, pneumothorax, alveolar-Interstitial Pathology and consolidation by LUS and CXR for detecting the sensitivity and specificity of each modality with CT chest done as a reference in cases of doubt. Results The comparison between Sensitivity and Specificity of US finding was (86.8% for parenchymal disease, 100.0% for pleural diseases) and (100.0% for parenchymal diseases, 94.7% for pleural diseases) respectively while CXR finding was (65.8% for parenchymal diseases, 75.0% for pleural diseases) and (58.3% for parenchymal diseases), (92.1% for pleural diseases) respectively among studied patients. Conclusion Transthoracic ultrasound is valuable for the evaluation of a wide variety of chest diseases, particularly mechanically ventilated patient. The advantages of low cost, bedside availability and no radiation exposure have made ultrasound an indispensable diagnostic tool in ICU. Lung ultrasound is more sensitive than chest x-ray in diagnosis and follow up chest diseases) PNX, alveolar-interstitial syndromes, parenchymal consolidations, and pleural effusion (LUS is an interesting medical method that is complementary to bedside CXR and reduces the need to use a CT scan.