Background <p>Pneumothorax is a common complication after CT-guided transthoracic lung biopsy. Pneumothorax is most often diagnosed with a conventional chest X-ray after a two-hour observation period. Ultrasound has been shown to be superior to conventional X-rays in diagnosing pneumothorax in some settings and it can be repeated without radiation exposure.</p> Purpose <p>The aim of the study was firstly to explore the sensitivity and specificity of thoracic ultrasound compared to conventional chest x-ray following CT-guided transthoracic lung biopsy, and secondly to investigate the dynamic changes of pneumothorax size using ultrasound.</p> Methods <p>This prospective study was conducted at Odense University Hospital in the Department of Radiology. Adult patients undergoing CT-guided lung biopsy were eligible for inclusion. A total of 26 patients were included in the study and were scanned five times in sitting upright and supine positions during the two-hour observation time using the FLUS Protocol.</p> Results <p>Pneumothorax was diagnosed via chest x-ray in 11 (42%) patients. With the patient in an upright position, the ultrasound had a sensitivity of 63.6% and a specificity of 93.3%. These numbers rose to 72.3% and 100% in the supine position. Monitoring the pneumothoraces showed a slight increase in size during the two hours.</p> Conclusions <p>Ultrasound can be used to diagnose pneumothorax after CT-guided lung biopsy. All pneumothoraxes that were identified by ultrasound were detected within the first 30&#xa0;min. The dynamic changes of pneumothoraxes showed that the size of the pneumothorax did not increase to a level where the patient needed intervention or admittance.</p>