Purpose: To evaluate the effectiveness and safety of tadalafil treatment for hypertensive disorder of pregnancy (HDP).Materials and methods: In an open-label, randomized clinical trial, singleton pregnancies with HDP between 20 and 33 weeks of gestation were randomized to take 20 mg oral tadalafil every day (tadalafil treatment group) or no drug (conventional treatment group). The primary outcome was prolongation of pregnancy from randomization to delivery. However, this article primarily focuses on the safety assessments performed in the tadalafil treatment for HDP population, because the safety of using PDE5 inhibitors as therapeutic agents for fetal growth restriction (FGR) has been a problem worldwide.Results: From October 2016 to March 2018, 28 patients were randomized to each group and two cases were excluded (tadalafil treatment group: 12 cases; conventional treatment group: 14 cases). The significant adverse events related to tadalafil did not occur in the tadalafil treatment group. Among maternal adverse events, specifically with regard to headaches, there were significant differences between the two groups (0% in tadalafil group versus 43% in conventional treatment group; p = .02). There was no difference in the prolongation period of pregnancy that served as primary outcomes in both the groups (17.5 d in tadalafil group versus 16.5 d in conventional group, p = .96). The significant adverse events occurred at the same frequency as between the conventional treatment group and the tadalafil treatment group. And, maternal headache decreased significantly in the tadalafil treatment group.Conclusions: Tadalafil treatment is safe for pregnant women with HDP. Moreover, tadalafil did not prolong the gestational period in pregnant women with HDP.