C-pattern reflects a variety of causes such as atelectasis (eg, resorptive, compressive, and passive), consolidation, contusion, infection, mass, and pulmonary embolism. It is often difficult to identify a cause of C-pattern with critical care ultrasonography alone. We anticipated that C-pattern in the form of compressive atelectasis was associated with pleural effusion in the posterolateral zone. In an effort to differentiate effusion-induced compressive atelectasis from consolidation or other causes of C-pattern, we created a variable, C-pattern without significant pleural effusion (< 20 mm), in zone 5. C-pattern without significant pleural effusion in zone R5 was seen in 14 patients (24%) in the group with CPE, 16 (38%) in the group with ARDS, and 10 (31%) in the miscellaneous group (P = .304). C-pattern without significant pleural effusion in zone L5 was seen in 16 patients (27%) in the group with CPE, 12 (29%) in the group with ARDS, and 10 (31%) in the miscellaneous group (P = .94). These two variables were included in the multivariate analysis; however, they were not statistically significant for differentiating CPE from ARDS.