The objective of this study was to evaluate the effectiveness of sublingual isosorbide dinitrate (ID) in the acute control of hypertension in patients with severe preeclampsia. A randomized, double-blind clinical trial with a preinclusion period of response to hydration and rest was performed in 60 patients with severe preeclampsia. The treatment group (30 patients) received 5 mg of sublingual ID, a dose that could be repeated on a second occasion. The control group consisted of 30 patients who received placebo. Both groups continued with rest and hydration during the study. Maternal blood pressure and fetal heart rate were measured in all patients every 10 min until 1 h was completed. Treatment was considered effective when the diastolic blood pressure could be reduced to between 80 and 100 mm Hg. ID was effective in 56.6% of the patients in the first 10 min after its administration and in 96. 6% from 40 to 60 min. In the treatment group the systolic and diastolic blood pressures dropped gradually from 171 +/- 13 and 113 +/- 8 mm Hg, respectively, to 136 +/- 14 and 87 +/- 9 mm Hg. In the placebo group, blood pressure was not reduced. There was no significant difference in fetal heart rate between both groups throughout the study. In conclusion, ID was shown to be effective in the acute control of hypertension in patients with severe preeclampsia, where high blood pressure persists after a period of rest and hydration.