['Transient tachypnea of the newborn (TTN) is a benign disorder with a variable clinical course that often leads to hospitalization. The aim of this study was to assess and validate the relationship between the serum cystatin C level and symptom duration in infants with TTN.', 'Forty newborns presenting with TTN and who had undergone serum cystatin C (Cys C) tests on the first day of admission to the Kyung Hee University Hospital (Seoul, Korea) from 2009 to 2013 were included. The serum Cys C level, creatinine (Cr) level, estimated glomerular filtration rate (eGFR), and tachypnea duration were correlated retrospectively.', 'The median gestation period was 37.8\xa0±\xa03.8 weeks and the mean birth weight was 3.2\xa0±\xa00.4\xa0kg. Tachypnea duration was 3.3\xa0±\xa02.0 days. Serum Cys C and Cr levels were 1.7\xa0±\xa00.2\xa0mg/L and 0.8\xa0±\xa01.2\xa0mg/dL, respectively. Tachypnea duration was significantly positively correlated with the serum levels of Cys C and significantly negatively correlated with Cys C-based eGFR (p\xa0=\xa00.016), but was not significantly correlated with the serum Cr level or Cr-based eGFR. When tachypnea duration was compared between infants with Cys C level <1.6\xa0mg/L (n\xa0=\xa015; Group A) and infants with Cys C level\xa0≥\xa01.6\xa0mg/L (n\xa0=\xa025; Group B), the symptom duration was significantly shorter in Group A infants (p\xa0=\xa00.011).'] Does decreased Cystatin C-Estimated Glomerular Filtration Rate be Correlated with Prolonged Hospital Stay in Transient Tachypnea of Newborn Infants?