Unstable angina usually responds to medical management. Unfortunately, a third of patients with this condition may be expected to have severe angina in the ensuing year. We tested the ability of thallium 201 imaging with submaximal exercise to identify patients who have a poor clinical prognosis after an episode of unstable angina. In all, 37 patients were evaluated with (201)TI before hospital discharge and were assessed for severity of angina over the next 12 weeks. Of these, 20 had New York Heart Association class I or II angina during follow-up, 4 of whom had defects on (201)TI imaging that showed redistribution. In 17 patients class III or IV angina developed or they suffered an acute myocardial infarction after hospital discharge, and 15/17 had reversible defects on (201)TI imaging after submaximal stress (P<.001). (201)TI scintigraphy frequently identifies areas of ischemia with only submaximal exercise in patients whose unstable angina has responded to medical therapy.