The ability to distinguish intrathoracic from intraabdominal fluid collections has important therapeutic implications. In the setting of peridiaphragmatic fluid collections CT accurately distinguishes between pleural and subphrenic fluid collections in most cases. Subpulmonic effusions represent a potential pitfall to CT diagnosis, however, because the atelectatic basilar lung segments appear on axial CT sections as a curvilinear band density that simulates the hemidiaphragm. Fluid found anterior to this "pseudodiaphragm" will erroneously be diagnosed as subphrenic unless the atelectatic lung is recognized as such. New CT observations are reported that allowed confident diagnosis in 18 consecutive cases of subpulmonic effusions. The atelectatic lung was distinguished from the diaphragm because the atelectatic band appeared thickened (17 of 18 cases), tapered laterally (14 of 18), was interrupted rather than continuous (17 of 18), and could be followed in contiguous cephalad sections into lung that was confidently recognized by the presence of gas bubbles or air bronchograms (12 of 18).