Sixteen patients under treatment for unstable angina (UA) were subjected to 24-hour Holter monitoring after having been asymptomatic for at least 12 hours. 12-lead ECGs were obtained every 4 hours and when anginal pain supervened. Six patients (37.50%) had no evidence of ischemia in the Holter recordings and in the 12.lead ECGs and reported no anginal pain; five (31.25%) reported no pain but had evidence of ischemia in the Holter recordings and five (31.25%) reported anginal pain and had evidence of ischemia both in the Holter recordings and in the 12-lead ECGs. The probability for a 12-lead ECG to record an episode of silent myocardial ischemia (SMI) was only 1.54 x 10(-5). Medical treatment using conventional criteria was successful in 11 patients (68.75%). If the results of Holter monitoring are considered, the success rate was 37.50% (6 out of 16 patients free from ischemia). We conclude that as Holter monitoring reveals the episodes of SMI, it is a better means for assessing the results of medical treatment.