Water balance studies were performed in 12 full-term hyperbilirubinemic newborn infants receiving phototherapy. The results were compared with 14 control and 10 jaundiced infants who did not receive phototherapy. In phototherapy infants, the respiratory rate, stool water loss, and insensible water loss were significantly greater than the control and jaundiced infants who did not receive phototherapy. These results emphasize the importance of careful calculation of water intake to maintain positive water balance in infants receiving this form of therapy.