The purpose of this study was to evaluate the antihypertensive efficacy of captopril (C) and its correlation with body mass index (BMI), age and initial blood pressure (BP) in hypertensive crises. The time of the beginning of the antihypertensive effect of C and its side effects were evaluated. Twenty patients, aged between 22 and 59 years, were included in this investigation. BP was measured after 15 min of rest; age, weight and height were recorded and BMI was calculated using the standard formula (Kg/m2). Patients were not taking antihypertensive medications. After the placebo was given to the patients, BP was measured at 30 and 60 min. Sublingual captopril 25 mg was administered and BP was again measured at 5, 10, 15, 30, 60, 120 y 240 min. There was not statistically significant difference between the values of BP before and after placebo. A significant decrease was observed post C, from 182.6/123.6 to 174.6/117.3 (p:SN/p < 0.02) in 10 min., to 169.9/114.9 (p < 0.04/p < 0.001) in 15 min., to 167.7/112.2 (p < 0.02/p < 0.0001) in 30 min., to 164.3/108.8 (p < 0.002/p < 0.0001) in 60 min., to 156.9/106.5 (p < 0.0001/p < 0.0001) in 120 min., to 161.3/107.0 (p < 0.0002/p < 0.0001) in 240 min. In 15 patients there was a decrease in diastolic BP > or = 5 mmHg, 10 min. post C. No correlation was found between decrease in BP and age, nor with BMI. Pearson r correlation index between the decrease in systolic BP and initial systolic BP was 0.63 (p < 0.003) and between the decrease in diastolic BP and initial diastolic BP, 0.59 (p < 0.005). Secondary effects were mild and well tolerated. In conclusion, C effectively reduced BP in hypertensive crises. Because the efficacy of C, its rapid onset of action and minimal side effects, sublingual Captopril should be considered a first line drug for hypertensive crises.