BACKGROUND AND OBJECTIVE: Using light-emitting diodes during conventional phototherapy, it is possible to reduce the distance from light source to infant, thus increasing light irradiance. The objective of this study was to search for a “saturation point” (ie, an irradiation level above which there is no further decrease in total serum bilirubin [TsB]). This was a prospective randomized study performed in the NICU of Aalborg Hospital, Denmark. METHODS: One hundred fifty-one infants (gestational age ≥33 weeks) with uncomplicated hyperbilirubinemia were randomized to 1 of 4 distances from the phototherapy device to the mattress (20, 29, 38, and 47 cm). TsB was measured before and after 24 hours of phototherapy and irradiance every eighth hour. Main outcome was 24-hour decrease of TsB expressed in percent, (∆ TsB0–24, difference between TsB0 and TsB24 [%]). RESULTS: A highly significant linear relation was seen between light irradiance and ∆ TsB0–24 (%) (P < .001): when the irradiance increased from 20 to 55 μW/cm2/nm, ∆ TsB0–24 (%) increased from approximately 30% to 50%. In addition, smooth regression showed no tendency for ∆ TsB0–24 (%) to level off as irradiance increased. ∆ TsB0–24 (%) was negatively correlated to birth weight and positively to formula volume. Average weight gain during phototherapy was 1%, independent of light irradiance. CONCLUSIONS: By using light-emitting diodes, we found a linear relation between light irradiance in the range of 20 to 55 μW/cm2/nm and a decrease in TsB after 24 hours of therapy, with no evidence of a saturation point.