The antihypertensive efficacy of sustained-release isradipine administered once daily compared to the immediate-release formulation administered twice daily was assessed by ambulatory blood pressure (BP) monitoring in a double-blind randomized crossover study in 76 mild-to-moderate hypertensive patients. Conventional BP and heart rate parameters were evaluated after a 4-week placebo period and patients qualified for entry if sitting diastolic BP was between 95 and 114 mm Hg. Ambulatory BP monitoring was measured at baseline and after active treatment with both formulations. The 2 regimens induced a significant and almost identical reduction (p less than 0.001) in the mean 24-hour BP without affecting heart rate. Isradipine was more effective in patients whose clinical hypertension was confirmed by ambulatory BP monitoring (35) than in patients who remained normotensive by ambulatory BP monitoring criteria (41). The isradipine-treated ambulatory hypertensive group experienced significantly greater decreases in BP during 24-hour, work, awake and sleep periods than did the ambulatory normotensive group. These data suggest that sustained-release isradipine has a sustained antihypertensive effect throughout 24 hours comparable to that of isradipine given twice daily and may improve compliance with long-term treatment. In addition, the results confirm the usefulness of ambulatory BP monitoring in determining truly hypertensive patients likely to respond to drug administration.