Primary spontaneous pneumothorax (PSP) is a common cause of presentation to emergency departments and subsequent hospitalization. Patients with large PSP are treated with tube thoracostomy (TT) and followed up with x-rays. In this study, we investigated the efficiency of bedside ultrasound and compared it with x-ray imaging for the clinical follow-up of PSP patients treated with TT.This is a prospective observational study. After ethical committee approval and written informed consent were obtained, patients who were treated with TT because of PSP were screened. In the follow-up of these patients, a bedside lung ultrasound (BLUS) was performed before every chest x-ray by an emergency physician experienced in performing BLUSs. The performance of BLUSs in detecting free air in the pleural cavity was compared statistically with that of x-rays.Sixty-two patients were enrolled in the study. In total, 166 BLUSs and x-rays were compared. The sensitivity of BLUS was 95.65% (85.20-99.50), specificity was 100% (79.40-100.00), positive predictive value was 100% (92-100), negative predictive value was 88.90% (65.30-98.60), and the area under the curve was 0.99 (0.974-1.000; P = 0.001) for detecting air in the pleural cavity. These results showed that there was no statistically significant difference between BLUS and x-ray methods for detecting air in the pleural cavity.Our study revealed that BLUS can be safely used for the follow-up of PSP patients treated with TT to determine if air is present in the pleural cavity. Further studies are needed.