Background: Subependymal/intraventricular hemorrhage (SE/IVH) is a frequent cause of disability and mortality, Methods: This is a prospective, randomized, double-blind study which included 100 pregnant Mexican women who need to interrupt their pregnancy within 28 - 32 weeks of gestation, One group was given a single dose of intravenous (IV) phenobarbital 10 mu g/kg (phenobarbital group, n = 50), and the other was provided with diluted distilled water (control group). Measurements of phenobarbital serum concentrations were taken by both mother and newborn, and head sonograms were applied during the first 24 hours, at the 3rd and 7th days of life. Results: The sample was made up of 42 newborns in the phenobarbital group, and 46 in the control group; the newborns had phenobarbital levels of 11.5 5.7 g/mu l at birth, and of 9.5 +/- 5.9 g/mu l 24 hours later. SE/IVH was found in 12 patients from the phenobarbital group and in 29 from the control group (p<0.005), the first group were 11 mild SE/IVH (2 grade I, and 9 grade II), and 26 in the control group (4 grade I, and 22 grade II), p<0.005. Severe hemorrhages mere similar between groups, A larger frequency of SE/IVH was found in the newborn group which received mechanical ventilation (p = 0.0008). Conclusions: Prenatal phenobarbital can reduce the SE/IVH frequency in premature infants younger than 32 weeks at birth, Its main effect could be shown in patients with mechanical ventilation.