Conventional phototherapy systems that simultaneously irradiate the front and the back of the baby lower the serum bilirubin level more rapidly than one-sided systems, but they are impractical. Fiberoptic phototherapy makes it easy to administer conventional phototherapy from above while the infant lies on a fiberoptic phototherapy blanket. Newborns with birth weights less than 2500 g were randomly assigned to receive either single (n = 37) or double (n = 33) phototherapy. The groups were similar in clinical and laboratory characteristics. After 18 hours of therapy the serum bilirubin concentration declined by 31 ± 11% in the double and 16 ± 15% in the single phototherapy group (2.9 ± 1.1 vs 1.6 ± 1.4 mg/dL), and the difference in the total serum bilirubin levels after 18 hours of therapy was significant (double phototherapy group 7.1 ± 2.7 mg/dL vs single phototherapy group 8.2 ± 2.6 mg/dL). After 18 hours of treatment the serum bilirubin level was less than the phototherapy threshold level in 26 of 37 single phototherapy patients vs 32 of 33 double phototherapy patients. Double phototherapy was well tolerated. It is concluded that this type of double phototherapy is more effective than single phototherapy in low birth weight newborns. Double phototherapy may be useful when it is necessary to reduce an elevated serum bilirubin level as rapidly as possible or when the bilirubin level is rising with single phototherapy.