Sex differences in cerebral blood flow (CBF) values have been demonstrated in adults but not in newborns. This study evaluated the influence of sex, intrauterine growth, and need of mechanical ventilation on resting cerebral blood flow in preterm neonates. Sixty-eight preterm infants with gestational ages of less than 34 weeks and birth weights of less than 1,500 gm were enrolled into the study. Cerebral blood flow was measured by the noninvasive intravenous xenon 133 method 3 times. Measurements were classified into 3 groups: group 1: measurement at 2-36 hours (n = 46); group 2: measurement at 36-108 hours (n = 39); and group 3: measurement at 108-240 hours (n = 41). In all 3 groups, the CBF in girls was significantly lower than in boys (group 1: 11.5 +/- 2.8 ml/100 gm/min vs 14.0 +/- 4.1 ml/100 gm/min; group 2: 13.4 +/- 2.9 ml/100 gm/min vs 16.3 +/- 4.3 ml/100 gm/min; group 3: 12.9 +/- 3.2 ml/100 gm/min vs 15.3 +/- 3.1 ml/100 gm/min). In group 1, the CBF in neonates requiring mechanical ventilation was significantly lower (P < .05) than in patients who were spontaneously breathing (11.5 +/- 3.7 ml/100 gm/min vs 14.2 +/- 3.1 ml/100 gm/min), and the CBF in neonates who were too small for gestational age was significantly higher (P < .005) than in children with appropriate intrauterine growth (16.1 +/- 4.1 ml/100 gm/min vs 11.5 +/- 2.6 ml/100 gm/min). It is concluded that in preterm neonates CBF is substantially affected by sex, intrauterine growth retardation, and the need of mechanical ventilation.