The antihypertensive effect of isradipine compared with placebo was assessed in 28 male patients (aged 40-64 years) with mild-to-moderate essential hypertension. After withdrawal of all previous antihypertensive treatment, these patients were entered into a 4-week placebo period followed by randomization to receive double-blind, for 8 weeks, either placebo (n = 14) or isradipine at 1.25-2.5 mg twice daily (n = 14). Twenty-four-hour ambulatory blood pressure was measured by Accutracker at the end of the placebo period and at the end of active treatment. In the isradipine group, both systolic and diastolic blood pressures decreased significantly (p less than 0.0001) whereas blood pressure increased in those taking placebo. In addition, isradipine as monotherapy controlled blood pressure throughout the day and night, and especially during the early morning, in these patients.