Isradipine, a new antihypertensive dihydropyridine calcium antagonist, was evaluated for its efficacy, tolerability, and safety in 91 ambulatory patients who had mild-to-moderate hypertension. The design of the present study included a two-week wash-out period after confirmation of disease, followed by 12 weeks of active treatment with 2.5 mg isradipine twice daily. Patients were switched from other antihypertensive drugs, mainly diuretics and beta-blockers. The dose of isradipine remained virtually unchanged throughout the study and resulted in a mean decrease of 22 mm Hg in systolic blood pressure (SBP) (P less than .00001) and 19 mm Hg in diastolic blood pressure (DBP) (P less than .00001). Heart rate was unchanged (difference of -1 beats/min), as was the mean body weight of the study patients. Isradipine was generally well tolerated. Side effects were few and, when present, tended to diminish and eventually disappear during the treatment period. All of the clinical laboratory parameters tested and electrocardiograph intervals remained unchanged. In conclusion, these results indicate that isradipine is a novel drug which is highly effective and well tolerated in the treatment of mild to moderate hypertension in this group of patients.