Objective: This study aimed to estimate the effectiveness of L-arginine for preventing preeclampsia in high-risk pregnancy. Methods: We performed a randomized, double-blind, placebo-controlled, clinical trial in patients with high-risk factors for preeclampsia. Fifty subjects received L-arginine, beginning from the 20th week of gestation. An additional 50 patients received homologated placebo. Results: The placebo group had a higher number of cases of preeclampsia (11/47) compared with the L-arginine group (3/49, P = 0.01). Birth weight was higher in the L-arginine group and there was a smaller number of preterm births (P = 0.03). Conclusion: L-arginine is effective for preventing preeclampsia.