Background The palpation method is widely used in clinical practice to identify the puncture site of combined spinal-epidural (CSE) blocks, but it is usually difficult to accurately locate the puncture site in obese parturients. Accurate identification of the puncture site is crucial for successful CSE block. The objective of this study was to evaluate the impact of ultrasound imaging on the success rate of CSE puncture in obese parturients. Methods Sixty obese parturients with a body mass index >= 30 kg/m(2) who were scheduled for caesarean section were randomized into two equal-sized groups for location of the puncture site: an ultrasound group and a palpation group. The success rate of puncture at the first puncture site, the number of puncture attempts, duration of CSE procedure, time taken to determine the puncture site, and the depth of the epidural space were compared between groups. The frequencies of complications such as puncture site hemorrhage, neurological damage, and inadvertent dural puncture were also studied. Results There were no differences in age, body weight, height, body mass index, or gestational age between the two groups. The success rate of puncture at the first puncture site was significantly higher in the ultrasound group than the palpation group (100.00% vs. 70.00%, P=0.004). The number of puncture attempts was significantly lower in the ultrasound group than the palpation group (chi(2)=6.708, P=0.035). The time taken for determining the puncture site was (0.30 +/- 0.12) minutes in the palpation group and (2.60 +/- 0.61) minutes in the ultrasound group (P<0.001). The duration of CSE procedure was (7.67 +/- 1.52) minutes in the palpation group and (9.37 +/- 1.35) minutes in the ultrasound group (P<0.001). The depth of the epidural space was similar in both groups (P=0.586). Puncture site hemorrhage was observed in 6 (20.00%) patients in the palpation group and 2 (6.67%) patients in the ultrasound group (P=0.255). Conclusions Ultrasound imaging improves the rate of successful puncture at the first puncture site and decreases the number of puncture attempts. It facilitates CSE puncture in obese parturients. Chin Med J 2012;125(21):3840-3843