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Background and Objectives: Serratus plane block is performed for analgesia of the anterior chest wall. However, there has been no study concerning the appropriate volume for this block. This prospective randomized controlled study assesses the dermatomal spread and analgesic effects of serratus plane block.Methods: Ultrasound-guided serratus plane block was performed for breast cancer surgery. The patients were randomly assigned to receive 20 or 40 mL of 0.375% ropivacaine. The primary end point was the number of affected dermatomes as assessed by cold test and pinprick test 20 minutes after the block procedure. Secondary end points were the time until the first postoperative analgesic rescue, adverse effects, and complications.Results: The number of affected dermatomes assessed by the cold test for patients receiving 40 mL of 0.375% ropivacaine was significantly larger than that for patients receiving 20 mL (P = 0.002; 6 [5-7] vs 4 [3-4] dermatomes). Similarly, with the pinprick test, the affected area was larger for the 40 mL group than for the 20 mL group (P = 0.009; 4 [2-6] vs 2 [1-3] dermatomes). There were no differences between the 2 groups in secondary end points.Conclusions: Ultrasound-guided serratus plane block spread in the craniocaudal direction is more widespread with 40 mL than with 20 mL of 0.375% ropivacaine. The time until the first postoperative analgesic rescue dose was not extended by a larger volume of injection.Clinical Trials Registration: UMIN Clinical Trials Registry (identifier UMIN000016549). [ABSTRACT FROM AUTHOR]