An interim analysis of patient compliance is reported in 234 hypertension outpatients who were entered into a large-scale, open, crossover, comparative study between a new-generation calcium antagonist, amlodipine (5 mg, once daily), and nifedipine SR (20 mg, twice daily). An analysis was also performed on 84 outpatients with stable angina pectoris, who were included in an open, parallel study and received the same dosing regimen of either amlodipine or nifedipine SR as the patients in the hypertension arm of the study. In the hypertensive patients, there were significant differences in favor of amlodipine, using all four methods to assess patient compliance. In the angina patients, a significant difference between the groups in favor of amlodipine was only found using the "correct dosing" and the "timing compliance" methods. With the traditional pill-counting and also the "taking compliance" methods, there was no observed difference in compliance between the two groups. It was concluded that, in terms of patient compliance, once-daily amlodipine was markedly superior to twice-daily nifedipine in the crossover study involving the hypertension patients. Amlodipine was also better tolerated than nifedipine. In the angina arm of the study, patient compliance was again better with amlodipine than with nifedipine, but there was no difference observed in the levels of tolerance between the two therapies.