The objective of this study was to characterize factors affecting the success rate of nurse-performed ultrasound (US)-guided peripheral vein cannulation in difficult access patients. We prospectively enrolled patients who had two previous unsuccessful i.v. attempts. Nurses were trained in US-guided cannulation using a 7.5-MHz linear probe. The following characteristics were recorded: 1) reason for difficult access, 2) which upper arm veins could be sonographically visualized, 3) cannulation success, and 4) one- or two-person technique used. One hundred patients were enrolled. The cannulation success rate was 63%. Cannulation of the basilic vein was successful in 39 of 55 attempts (71%), whereas the success rate for the brachial vein was only 19 of 46 attempts (41%). The reason for difficult access and the one- or two-person technique did not affect success rate. The basilic vein was the best choice for cannulation, and the one-person technique was as successful as the two-person technique.