A controlled study involving 96 babies weighing under 2,500 gm was done by placing 47 completely exposed infants, except for diapers, under environmental lights averaging 90 footcandles and 49 fully clothed infants under lights averaging 10 footcandles. A significantly lower serum bilirubin occurred in the light group from the second to the sixth day. Only three babies developed indirect bilirubin levels over 15 mg/100 ml in the light group against 14 in the control group. Exposure to this intensity of environmental lighting seems to be of much value in the routine treatment of premature infants for the prevention of hyperbilirubinemia.