Thoracic sonography has been advocated as being complementary to small-bore catheter drainage in pleural effusions. However, it is not known whether the initial sonographic appearances of empyemas or complicated parapneumonic effusions (CPPEs) can predict the outcomes of small-bore catheter drainage for these pleural insults. This retrospective study investigated the outcomes of patients who had been diagnosed with empyema and CPPE and had received ultrasound-guided small-bore catheter (size from 12F to 16F) drainage in a tertiary university hospital from September 2005 to August 2007. Patients were excluded when empyemas or CPPEs were traumatic, they were less than 18 years old or their charts were incomplete. We evaluated 141 small-bore catheters in 70 patients with empyemas and 71 patients with CPPEs over a two-year period. The mean age was 58+/-15 y and the male gender was more frequent (112 men, 79%). The overall successful rate of small-bore catheter drainage in empyemas or CPPEs was 63% (89/141). The sonographic appearances of these empyemas or CPPEs exhibited a complex septated pattern in 57% (81/141) of patients and a complex nonseptated pattern in 43% (60/141) of patients. The success rate in a complex nonseptated sonographic pattern was significantly higher than in a complex septated sonographic pattern (48/60, 80% vs. 41/81, 51%, respectively; p=0.001). Moreover, patients with complex septated sonographic patterns also had higher intensive care unit admission rates compared with nonseptated sonographic patterns (22/81, 27%, vs. 8/60, 13%, respectively; p=0.0047), as well as infection-related mortality rates (17/81, 21% vs. 4/60, 7%, respectively; p=0.018). The appearance of sonographic septation is a useful sign to help predict the outcome of small-bore catheter drainage in cases of empyemas or CPPEs. Patients with a complex septated sonographic pattern have a poorer prognosis for a successful outcome, higher ICU admission rate and a higher mortality rate.