To determine if the phosphodiesterase type 5 inhibitor sildenafil prolongs pregnancy in women with preeclampsia.  Women with preeclampsia at gestational ages 24–34 weeks were recruited from nine hospitals in the UK, and randomly assigned to sildenafil citrate or placebo. Medication was increased every 3 days from 20 mg three times daily (tid), to 40 mg, and 80 mg tid. The primary endpoint was prolongation of pregnancy from randomisation to delivery (days). Secondary endpoints were markers of maternal disease and cord pH at delivery and infant weight. Details of all adverse events were also collected. Plasma samples were taken to establish pharmacokinetic information. Data analysed on a modified intention to treat analysis. The study had a power of >95% to detect a difference of 5 days.  Of 35 women, 17 were allocated to sildenafil and 18 to placebo. There was no difference in time from randomisation to delivery in the two treatment groups, with a median time of 4 days (range 1–15) in the sildenafil group and 4.5 days (range 1–30) in the placebo group. Sildenafil achieved maximum drug concentrations of 48 ng/ml, 88 ng/ml, and 271 ng/ml after 3 days of 20 mg, 40 mg and 80 mg tid, respectively. Conclusion: We have safely conducted a clinical trial of a drug not routinely used during pregnancy. Sildenafil in the escalating dose regimen 20–80 mg tid was well tolerated, with no increase in maternal or fetal morbidity or mortality but did not prolong pregnancy duration in women with preeclampsia. (ClinicalTrials.gov number, NCT 00141310).