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Summary: Background: Single injection ultrasound‐guided rectus sheath blocks are used for postoperative analgesia after midline abdominal incisions, but the ultrasonographic spread of medication posterior to the rectus muscle has not been investigated. Aims: The primary goal of this study was to determine the ultrasound‐measured medication spread superiorly and inferiorly after single injection ultrasound‐guided rectus sheath blocks and to calculate the incidence of cases with spread up to the subcostal margin and below of umbilicus. The secondary goals were to correlate the spread with the volume of medication administered and with the patient's age, weight, height, sex, and body mass index. Methods: Pediatric patients who underwent single injection ultrasound‐guided rectus sheath blocks and underwent data collection of ultrasonographic information on medication spread along the plane of the posterior rectus sheath were identified retrospectively from an acute pain service database at UPMC Children's Hospital of Pittsburgh. Results: Sixty‐eight children, 10.2 ± 4.2 (mean ± SD) years old, 41.7 ± 17.9 kg, 140 ± 24.6 cm, had complete data collection. Mean injected volume of ropivacaine was 8.3 ± 2.8 mL (right) and 8.2 ± 2.8 mL (left). The average spread was measured as 3.9 ± 1.4 cm (right) and 3.4 ± 1.3 cm (left) cranial to the umbilicus, and 1.5 ± 1.6 cm (right) and 1.6 ± 1.4 cm (left) caudal to the umbilicus. Complete spread up to the level of the subcostal margin was observed in 52.9% (n = 36; right) and 36.8% (n = 25; left) of cases. The spread correlated closely with the volume of injected ropivacaine (right: 0.556, P < 0.001; left: 0.541, P < 0.001). The spread below umbilicus was 70.6% (n = 48, right) and 80.9% (n = 55, left). There was a positive strong correlation (P < 0.001) between total medication spread and age (right: 0.608; left: 0.538), weight (right: 0.600; left: 0.540), and height (right: 0.593; left: 0.526). Conclusion: After single injection ultrasound‐guided rectus sheath blocks, incomplete cephalocaudal medication spread can be expected within the posterior rectus sheath. [ABSTRACT FROM AUTHOR]