The effectiveness of phototherapy in the management of neonatal hyperbilirubinemia in glucose-6-phosphate dehydrogenase (G6PD)-deficient infants was studied. "Prophylactic" phototherapy for six continuous days commencing from the first day of life was effective in preventing a significant rise in bilirubin levels in 12 G6PD-deficient infants in the first three days, during which period a rapid rise was observed in a control group of G6PD-deficient infants. The hemoglobin levels on the first and eighth postnatal days were comparable in both groups. "Therapeutic" phototherapy proved equally effective in reducing bilirubin levels in 24 infants with nonhemolytic hyperbilirubinemia and an equal number of infnats with hyperbilirubinemia associated with G6PD deficiency. Phototherapy was efficacious in the prevention or treatment of neonatal hyperbilirubinemia associated with G6PD deficiency; even if its use is prolonged it does not cause hemolysis in such infants.