The diagnostic capability of chest ultrasonography was assessed for use in evaluation of patients presenting with opacification of unilateral hemithorax on chest radiography. Hemithorax opacification provided an "ultrasonic window" for ultrasonic wave penetration to detect any underlying diseases. Fifty patients were included over a study period of 24 months. All had received both chest ultrasonography (US) and computed tomography (CT) examinations, and their underlying diseases had been confirmed by various methods. Comparisons between the findings of chest US and of CT were made in three compartments: pleura, parenchyma, and mediastinum. Diagnostic sensitivity and specificity of chest US was defined by using CT as the "gold standard." The ability of US to detect underlying diseases was comparable to that of CT in pleural and parenchymal lesions. But US was inferior to CT for demonstration of mediastinal lesions. The diagnostic sensitivity of US was 95.1% in pleural lesions, 82.8% in parenchymal lesions, and only 30% in mediastinal lesions. US missed 14 lesions demonstrated by CT (seven mediastinal lesions, five parenchymal tumors, and two pleural nodules), but it showed six lesions that CT had failed to detect (four focal pleural thickening and two parenchymal tumors). US-related procedures were performed (thoracentesis in 33 patients, pleural biopsy in 10 patients, and US-guided aspiration/cutting biopsy in 11 patients) with a high diagnostic yield and no complications. The conclusion was that chest ultrasonography is useful, particularly as a first-step examination for patients with total or nearly total opacification of the hemithorax.