Ultrasound guidance enables visualization of the needle insertion site for thoracentesis and paracentesis. The improved accuracy of needle placement using ultrasound may reduce risk of complications and their costs associated with these procedures. Using claims data from the Premier Perspective hospital database from January 1, 2007, through December 31, 2008, we conducted an observational cohort study examining the effect of ultrasound guidance on risk of pneumothorax among patients undergoing thoracentesis and on risk of bleeding complications after paracentesis. Patients at elevated risk of these outcomes for reasons beyond the procedure of interest were excluded. Adjusted risk of events was assessed using multivariate logistic regression controlling for patient and hospitalization characteristics. Hospitalization cost and length of stay (LOS) were estimated using multivariate ordinary least squares regression of log-transformed values. We analyzed 61,261 thoracentesis and 69,859 paracentesis patient records. Approximately 45% of these procedures were ultrasound guided. Pneumothorax occurred in 2.7% (n = 1,670) of patients undergoing thoracentesis. Of patients undergoing paracentesis, 0.8% (n = 565) experienced bleeding complications. After adjustment, ultrasound guidance reduced the risk of pneumothorax after thoracentesis by 19% (OR, 0.81; 95% CI, 0.74-0.90) and by 68% for bleeding complications after paracentesis (OR, 0.32; 95% CI, 0.25-0.41). Pneumothorax increased the total cost of hospitalization by $2,801 (P < .001) and LOS by 1.5 days (P < .001). Bleeding complications increased cost by $19,066 (P < .0001) and LOS by 4.3 days (P < .0001). The data indicate that ultrasound guidance is associated with decreased risk of pneumothorax with thoracentesis and of bleeding complications with paracentesis. These complications resulted in measurable increases in hospitalization costs and LOS.