This study will evaluate the use of acetaminophen in preterm infants when a patent ductus arteriosus (PDA) is of concern. We will perform two simultaneous prospective observational studies over a 3 year period. The first will be of infants with clinically significant PDAs beyond 14 days of life who are medically treated with acetaminophen as a means to avoid surgical ligation, and the second will be of infants who received acetaminophen for a PDA closure during the first 2 weeks of life as a result of ibuprofen, the current standard of care in our NICU, contraindication due to medical status.

Inclusion Criteria:

• Infants born at 23 0/7 to 29 6/7 weeks gestation, with PDA determined by medical team to need treatment and infant is beyond 14 days of life.

Exclusion Criteria:

• Congenital heart disease Pneumonia Pulmonary hypertension Pulmonary hemorrhage ALT and AST ≥ 2X the upper limit of normal (=AST> 150 U/L, ALT>90 U/L) Sepsis/meningitis Abnormal chromosomes Hydrops Major congenital malformations

Premature infants born between 23 0/7 weeks to 29 6/7 weeks gestational age.