Pneumothorax is considerably more common in HMD than in the general newborn population. Certain neonates with HMD possess a high risk of developing a pneumothorax, often under tension. The neonate at highest risk is a male, 1,500-2,300 grams at birth, 20-40 hours of age, with unrelenting respiratory distress and clinical deterioration, who previously demonstrated roentgenographic IIE, especially the variety which imparts "pseudo-clearing" of the lungs. In the presence of these findings the roentgenologist should predict the possibility of an impending pneumothorax, usually within the ensuing 24 hours. Although the mortality rate of a treated tension pneumothorax remains high, almost certain death will follow an untreated tension pneumothorax in HMD.