The appearance of pneumothorax on lateral radiographs obtained with the patient erect were reviewed in 100 patients (122 total examinations). A pneumothorax could be seen on the lateral projection in 89% of the examinations (109 of 122). The displaced pleural line was most frequently identified anteriorly or posteriorly and was less commonly identified at the lung apex or in a subpulmonic location. In 11 cases, an air-fluid level was the only recognizable finding of a pneumothorax on the lateral projection. Although in 27% of examinations (32 of 122) the pneumothorax was either not seen (11%) (n = 13) or was a subtle finding (16%) (n = 19), in 14% of examinations (17 of 122) the lateral projection provided helpful information to supplement the posteroanterior projection.