The efficacy and toleration of amlodipine (10 mg once daily) was compared with placebo in a four-week double-blind parallel group two-centre study in 38 patients who were receiving enalapril. Patients were moderate to severe hypertensives whose blood pressure was not adequately controlled by enalapril (5-10 mg once daily) (diastolic blood pressure greater than 95 mmHg after four weeks). Results are expressed as the mean difference, in baseline to final changes, between the two treatment groups. At the end of the double-blind period amlodipine patients had significantly greater decreases in supine (-19.4/-10.2 mmHg, P less than 0.001/P less than 0.001) and standing (-16.2/-10.2 mmHg, P less than 0.005/P less than 0.001) blood pressures than the placebo group. There was a greater fall (P = 0.005) in standing diastolic blood pressure at the UK centre (-17.1 mmHg) than in Denmark (-11.2 mmHg). However, pooling the standing diastolic blood pressure data from the two centres gave an overall significant mean treatment difference (-10.2 +/- 2.4 mmHg/P less than 0.001). There were no clinically or statistically significant changes in mean heart rate. Nineteen out of 20 patients in the amlodipine group compared to seven out of 18 patients on placebo, were assessed as improved by the investigator. Similarly 14 out of 19 patients receiving amlodipine were classified as responding to treatment (supine diastolic blood pressure less than 90 mmHg or reduction from baseline of greater than 10 mmHg after four weeks of double-blind therapy) in contrast to four out of 17 receiving placebo.(ABSTRACT TRUNCATED AT 250 WORDS)