Based on the known history of right upper lobe lung cancer and the characteristics of the chest radiograph (a well-rounded radiopacity with air-fluid level, clearly outlined minor fissure, and well-defined right costophrenic angle), the diagnosis of a lung abscess complicating a cavitating tumor was made. In some cases, it is difficult to differentiate lung abscess from empyema. Chest radiography alone is not useful when the empyema presents with an air-fluid level1; and although CT scan of the chest is valuable in the diagnosis of lung abscess, thoracic ultrasonography allows the treating physician to both follow the progression or regression of disease without repeated radiation and allow bedside therapeutic intervention with insertion of chest tube drainage if necessary.2 In this regard, chest ultrasonography is helpful to identify a lung abscess, typically described as a round or oval hypoechoic lesion with irregular outer margins with a hyperechoic ring3,4 (Videos 3, 4).1,5 Ultrasound findings suggestive of empyema include the split pleural sign, its wall characteristics, and the lesion shape. The split pleural sign is defined as thickened visceral and parietal pleura separated by empyema and is uncommon.5 Ultrasonographic wall characteristics (smooth vs irregular in empyema and lung abscess, respectively) and lesion shape (lenticular in empyema vs oval in lung abscess) are associated with high interobserver variability.5 Chen and colleagues5 found that septations and passive atelectasis were specific for empyemas but had low sensitivity (40% and 47%, respectively). The presence of color Doppler ultrasound signals in the pericavitary consolidation was the most useful and specific finding for identifying lung abscesses (sensitivity, 94%; specificity, 100%; positive predictive value, 100%; and negative predictive value, 94%) without interobserver variability (Video 11). This finding will not be present in patients with empyema, since compressive atelectasis causes a local reduction of blood flow.