Background The use of traditional techniques (such as landmark techniques,paresthesia and peripheral nerve stimulator) for upper-limb anesthesia has often been restricted to the expert or enthusiast,which was blind.Recently,ultrasound (US) has been applied to differ blood vessel,pleura and nerve,thus may reduce the risk of complications while have a high rate of success.The aim of this study was to determine if the use of ultrasound guidance (vs.peripheral nerve stimulator,(PNS)) decreases risk of vascular puncture,risk of hemi-diaphragmatic paresis and risk of Horner syndrome and improves the success rate of nerve block.Methods A search strategy was developed to identify randomized control trials (RCTs) reporting on complications of US and PNS guidance for upper-extremity peripheral nerve blocks (brachial plexus) in adults available through PubMed databases,the Cochrane Central Register of Controlled Trials,Embase databases,SinoMed databases and Wanfang data (date up to 2011-12-20).Two independent reviewers appraised eligible studies and extracted data.Risk ratios (OR)were calculated for each outcome and presented with 95％ confidence intervals (CI) with the software of ReviewManager 5.1.0 System (Cochrane Library).Results Sixteen trials involving 1321 adults met our criteria were included for analysis.Blocks performed using US guidance were more likely to be successful (risk ratio (RR) for block success 0.36,95％ CI 0.23-0.56,P ＜0.00001),decreased incidence of vascular puncture during block performance (RR 0.13,95％ CI 0.06-0.27,P ＜0.00001),decreased the risk of complete hemi-diaphragmatic paresis (RR 0.09,95％ CI 0.03-0.52,,P=0.0001).Conclusions US decreases risks of complete hemi-diaphragmatic paresis or vascular puncture and improves success rate of brachial plexus nerve block compared with techniques that utilize PNS for nerve localization.Larger studies are needed to determine whether or not the use of US can decrease risk of neurologic complications.