The nature of the dose response relationship of phototherapy for neonatal hyperbilirubinemia was studied in 110 infants divided into ten comparable groups; each group was subjected to phototherapy of different intensity. The response to phototherapy increases with increasing dose, but the rate of this response progressively decreases with increasing radiance till a "saturation point" is reached, beyond which no further increase in response occurs to further increase in radiance, i.e., an asymptotic regression was demonstrated. The minimal radiance at which phototherapy begins to be effective for neonatal hyperbilirubinemia was also determined. The rebound after cessation of phototherapy was similar in all groups of infants, despite the shorter duration of exposure required for the groups subjected to intense phototherapy.