 Phototherapy PT is widely used and assumed to be safe as well as effective in reducing total bilirubin TB levels. Our recent NICHD Network Trial showed that aggressive use of phototherapy reduces neurodevelopmental impairment NDI, but may increase deaths among ELBW infants. Among ventilator treated infants 750 g birth weight BW n 696, conservative Bayesian analyses using a neutral prior probability identified a 99 posterior probability that aggressive phototherapy reduced profound NDI but a 99 probability that it increased deaths relative to conservative phototherapy. The possibility that PT increases deaths among high risk infants is also suggested by the Collaborative Phototherapy trial performed in the 1970s, the only large RCT in which LBW infants were randomly assigned to receive PT or no PT. The relative risk for death among those randomized to PT relative to those randomized to no PT was 1.32 0.9-1.82 among all LBW infants and 1.49 0.93-2.40 among ELBW infants. These findings are consistent with a major increase in mortality but have been ignored because the p was 0.05, an error often made in ignoring important potential treatment hazards when power is limited. 