A double blind therapeutic trial of ethyl clofibrate as a preventive treatment of hyperbilirubinemia in preterm neonates was performed in neonates of gestational ages ranging between 31 and 36 weeks. Forty-six children were given the treatment and 43 a placebo. A single 100 mg/kg dose of ethyl clofibrate was administered orally, between the 24th and the 48th hour of life. Significant results in the treated neonates are as follows: a lesser intensity of jaundice from the 48th hour of treatment; a lesser need for repeated bilirubinemia assay for the control of evolution and a lesser use of phototherapy if the serum concentration of clofibric acid is above or equal to the 140 micrograms therapeutic level before the 24th hour of treatment. The analysis of results also shows that the therapeutic clofibric acid serum level is reduced in 66% of neonates of relatively high gestational ages (34-36 weeks) and in 33% only of neonates of lower gestational ages (31-33 weeks). This study, added to the previous therapeutic trial performed in at term neonates, shows the efficacy of clofibrate in the preventive treatment of hyperbilirubinemia in preterm neonates. Further studies will allow to define the exact dosage according to gestational age.