To define X-ray signs of restrictive pneumothorax, the authors analyzed X-ray films of 225 newborns and babies of the first 3 months of life who were examined for respiratory disorders and acute respiratory diseases. Restrictive pneumothorax was detected in 19 (8.44%) infants. In 14 patients, gas was present in limited areas of various portions of the pleural cavity, without causing collabation of the lung and displacing the mediastinum, which makes diagnosis difficult. Moreover, the thoracic superimposition of the border of the scapula, soft tissue of the shoulder, hypoventilation of the lower lobe simulated restrictive pneumothorax in 38 children. Examinations in the lateral position on the healthy side, X-ray made in milder modes than as accepted, dynamic studies were helpful in diagnosing restrictive pneumothorax. However, additional X-ray study increased radiation loads. The concordance of X-ray images of the available X-ray films on a UAP-2 device in 47 children enabled the presence of pneumothorax to be specified by the segregated outlines of the partially collabated lung and by the absence of a lung pattern outside. In hypoventilation of the lower lobe, a lung pattern was identified both in the hypoventilated and in the normal midlobe. The processing of X-ray films on the UAP-2 device enhanced the accuracy of restrictive pneumothorax diagnosis in infants and made it possible to do away an additional X-ray study, which reduced radiation loads.