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The objective of this study was to investigate the extent of dermatomal spread following an ultrasound-guided thoracic paravertebral block (PVB) when equal volumes of local anesthetic are injected at 1 versus 5 vertebral levels.Seventy patients undergoing a unilateral mastectomy were randomized to receive either single or multiple injections of a PVB under real-time ultrasound guidance using a parasagittal approach. The patients in the single-injection group received a PVB at T3–T4 level with 25 mL of 0.5% ropivacaine and 4 subcutaneous sham injections. Patients in the multiple-injection group received 5 injections of a PVB from T1 to T5 level. Five milliliters of 0.5% ropivacaine was injected at each level. Evaluation of the sensory block was carried out 20 minutes following the completion of the PVB.The median (interquartile range) dermatomal spread was not significantly different for the single-injection group (5 [4-6]) compared with the multiple-injection group (5 [5-6]), with a median difference of 0 segments (95% confidence interval, −1 to 0 segments; P = 0.22). The median time to performance of the single-injection PVB was shorter compared with the multiple-injection group (10 minutes), with a mean difference of −4 minutes (95% confidence interval, −6 to −3 minutes; P < 0.001).An ultrasound-guided single-injection PVB provides equivalent dermatomal spread and duration of analgesia compared with a multiple-injection PVB. The single-injection technique takes less time to perform and hence may be preferred over a multiple-injection technique.The trial was registered prospectively at ClinicalTrials.gov (NCT02852421) on July 15, 2016.