We prospectively examined 26 patients who were referred for ultrasound-guided thoracentesis, following at least one unsuccessful, clinically guided attempt. Sonographically guided thoracentesis was successful in obtaining fluid in 88% of patients. In addition, ultrasonography proved useful in suggesting or defining the cause for the initial unsuccessful attempt. Patients who have undergone an unsuccessful clinically guided thoracentesis and are referred for sonographic assistance represent a selected group who may have complicating factors not typically present during routine thoracentesis. Awareness of these potential complicating factors may facilitate the performance of ultrasound-guided thoracenteses.