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Background and Aims Background The Erector Spinae Plane (ESP) block is as a simple, ultrasound guided technique, widely used to provide thoracic analgesia. In the lumbar region, a similar technique has been described, but with less success. This may be due to anatomical differences between the fascial planes in the lumbar and thoracic regions, causing a different pattern of injectate spread. This study was performed to determine which structures are reached when fluid is injected into the lumbar ESP, to indicate the clinical applications of a lumbar ESP block.Methods Twelve patients were included in the study, performed between January to June 2023. All were being treated for chronic hip pain. Following a therapeutic hip block, a baseline MRI was performed. Saline (30 ml) was then injected on the contralateral side, into the ESP at the level of L4, using. an ultrasound guided, parasagittal, in-plane technique. A second MRI was performed after thirty minutes, and the spread of injectate analysed.Results Posterior spread within the erector spinae (ES) muscles, the location of dorsal rami of spinal nerves was seen in all patients. In n=4 (33%) patients, saline also spread anteriorly, to anatomical areas where ventral rami of spinal nerves or the lumbar plexus are located, and to the paravertebral space. Epidural spread was also seen in one of these four. Average craniocaudal spread of injectate was 140.3 mm (90–194 mm). Average number of vertebral levels covered was 5.75 (4–8), i.e. approximately 5.2 ml of injectate per vertebral level.Conclusions The spread of a bolus of fluid injected into the lumbar ESP tends to be posteriorly, implying usefulness for spinal surgery. Unpredictable anterior spread suggests that this may not be such a reliable type of block for surgeries requiring anaesthesia to ventral rami of spinal nerves or related anterior structures.