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Background/purpose: The purpose of this study was to examine the natural history and outcomes of prenatally diagnosed lung masses that appear to undergo complete regression before birth., Methods: An IRB-approved retrospective review was performed on 100 consecutive fetuses with a congenital lung malformation at a single fetal center. Prenatal and postnatal imaging as well as outcomes of vanishing fetal masses was analyzed and compared to those with persistent fetal masses., Results: Seventeen lesions (17%) became sonographically undetectable at 35.3 ± 2.3 weeks gestation. Vanishing fetal masses were associated with microcystic disease (100% vs. 69%, p=0.005) and a low initial congenital pulmonary airway malformation volume ratio (CVR; 0.31 ± 0.35 vs. 0.70 ± 0.66, p=0.002) when compared to those with persistent fetal lesions. Based on postnatal CT imaging and pathology data, 10.3% of all fetal masses completely regressed. The positive predictive value and negative predictive value of prenatal ultrasound for detecting lung malformations in late gestation were 96% and 43%, respectively. All infants with vanishing fetal lesions were asymptomatic at birth and were more likely to be managed nonoperatively (75% vs. 22%, p<0.0001) when compared to infants with persistent fetal masses., Conclusions: Vanishing lung lesions late in gestation are relatively common and are associated with a low CVR and microcystic disease., (Copyright © 2015 Elsevier Inc. All rights reserved.)