The Nifedipine-Total Ischemia Awareness Program was designed to evaluate the prevalence, prognostic implications and effect of therapy on painful and painless myocardial ischemic episodes in a nationwide study of patients with angina pectoris. Three hundred forty-eight patients with at least 2 anginal attacks/week while taking antianginal medications were enrolled at 53 participating centers between September 1, 1986 and March 31, 1988; 312 of the 348 patients formed the study group, while 36 patients formed the control group. At least 1 episode of ST-segment depression during two 48-hour periods of Holter monitoring was present in 136 of the 312 patients in the study group. In these 136 patients, there was a total of 372 episodes of ST-segment depression, of which only 69 (18%) were painful; 85% of the 136 patients had either painless episodes only or both painless and painful episodes. Despite apparently adequate antianginal therapy, 48 patients had greater than or equal to 3 episodes of ST-segment depression/48 hours of ambulatory electrocardiographic monitoring, and 38 patients greater than 60 minutes of ST-segment depression. After nifedipine was administered, there was a 23% reduction in the mean number of episodes of ST-segment depression (2.7 +/- 0.3 to 2.1 +/- 0.2, p less than 0.01). The most pronounced effects were found in the 48 patients with greater than or equal to 3 episodes of ST-segment depression and the 38 patients with greater than or equal to 60 minutes of total ischemic time.(ABSTRACT TRUNCATED AT 250 WORDS)