Sir,We report a case of a 45-year-old American Society of Anesthesiologists physical Status I female patient who underwent right-sided modified radical mastectomy for ductal carcinoma. The surgery was conducted under general anaesthesia with ultrasound (US)-guided pectoralis (PECS I and II) block for analgesia. Following anaesthesia induction, the patient's right arm was placed in an abducted position, and a pre-puncture scan of the right haemithorax was performed to visualise the anatomical details for the block. The block was then performed at the level of the 3rd rib. The needle recommended for the block was not available, thus a 22-gauge Quincke–Babcock (QB) needle was used, which was inserted at the target area by the out-of-plane technique. After confirmation of the needle placement, 20 ml of 0.25% bupivacaine was deposited for the PECS II block and 10 ml for the PECS I block. The surgery proceeded uneventfully, and at the end of the surgery, the trachea was extubated. Immediately after extubation, the patient developed difficulty in breathing with a respiratory rate of 34/min, use of accessory muscles and paradoxical movement of the chest. Auscultation of the chest revealed decreased air entry in the right upper zone. With a high index of suspicion of pneumothorax, urgent lung US was done. It revealed an absent lung sliding sign in the right lung upper zone in the 2nd to 4th intercostals spaces, corresponding to the area where our needle had been inserted. This was highly suggestive of pneumothorax and management was initiated with patient propped up and oxygen supplementation. The patient was shifted to the post-anaesthesia care unit where a chest X-ray was done, possible only after 45 min. It corroborated with the finding of the US, demonstrating the presence of pneumothorax on the right side, although small (visible rim <2 cm at the level of hilum as per the Rhea method). The pneumothorax resolved on conservative management over the next 8 h.US-guided pectoralis (PECS I and PECS II) block is a widely accepted technique for analgesia in patients undergoing breast surgeries.[1] However, as with any invasive procedure performed in close vicinity to the lungs, pneumothorax is a known complication. The chances of this complication are more when a spinal needle is introduced by the out-of-plane technique, where the needle enters perpendicular to the pleura; however, it is technically easier to perform and often used by beginners.[2] US, being an effective point-of-care tool, can be immediately used for the assessment and confirmation in such cases.[3] Sonographic diagnosis of pneumothorax requires the absence of lung sliding, absence of B-lines, and presence of lung pulse and the presence of lung point. It is known that these signs on US have a 93%–100% negative predictive value for pneumothorax.[45] We would thus like to highlight the utility of pre- and post-puncture US to detect potential complications associated with PECS block or with any other invasive procedure performed in the thoracic region.