Objective: To assess the benefit of early L-arginine administration in preeclampsia on the relative risk to fetal growth. Patients and methods: One-hundred women with preeclampsia were randomized to receive either L-arginine or placebo until the day of delivery. To evaluate the relative risk of intrauterine growth restriction (IUGR) and the effect of L-arginine on this process, 96 live singleton infants of women with preeclampsia (50 with treatment and 46 without treatment) were compared; these infants were also compared with a further 50 control infants of healthy women. Gestational age-related birth weight was compared using standard growth curves. Infants smaller than the 10th percentile were classified as IUGR. The Mann-Witney U-test, ANOVA, and chi-square test were used to evaluate statistically significant differences (P<.05) between the groups. Results: No significant differences were found between the groups with preeclampsia before randomization. Preeclampsia was associated with a 21% reduction in birth weight. The risk of IUGR was five times higher in infants born after preeclampsia without L-arginine therapy than in control pregnancies (RR = 5.0; 95%IC: 1.5-16.2) and was two times higher in infants born after preeclampsia with L-arginine therapy (RR = 2.0; 95% CI: 1.9-7.6). The fetal biophysical profile and Apgar score were significantly more favorable in the L-arginine group (P<.05). Conclusion: Fetal growth markedly improves with early L-arginine therapy in women with preeclampsia. © 2009 Sociedad Española de Ginecología y Obstetricia.