The NICHD Neonatal Research Network conducted a randomized trial of aggressive versus conservative phototherapy in babies with a birth weight of 1000 grams or less. Aggressive phototherapy was provided at a total serum bilirubin value of 5 mg/dL or higher in the first week and 7 mg/dL or higher in the second week. Conservative phototherapy was provided at a bilirubin value of 8mg/dL or higher for 501–750 gram infants and 10mg/ dL or higher for 751–1000 gram infants. The trial found no difference in the composite primary outcome of death or neurodevelopmental impairment. However, aggressive phototherapy lead to a lower rate of neurodevelopmental impairment at 18 – 22 months. Of concern, there was a statistically non-significant increase in mortality in babies 501 – 750 grams. Data from the trial were analyzed using a Bayesian analysis (in addition to frequentist analysis). Ventilated babies < 750 grams were found to have lower probability of impairment and profound impairment, but these benefits were offset by a higher probability of mortality. In the smallest and sickest NICU infants aggressive phototherapy may increase mortality while decreasing neurodevelopmental impairment.