Although the efficacy of long-term administration of antithrombotic agents in unstable angina has been established, short-term effects on myocardial ischemia are unknown. A retrospective analysis was performed in 47 patients undergoing three-channel continuous ST segment monitoring as part of a multicenter trial using esmolol in unstable angina, in which 20 patients received a continuous heparin infusion during the initial assessment of chest pain. Concomitant medications included calcium channel blockers, β-adrenergic blockers, nitrates, and aspirin in the majority of patients. Clinical variables between the heparin and no heparin groups were similar, except for fewer males and fewer total artery occlusions in the heparin group. No significant differences in the incidence or duration of ischemia were found in a 36 ± 16 hour monitoring period. Forty percent of the heparin group had 35 episodes of ischemia with a mean of 11 ± 10 minutes per episode and a total ischemic time of 48 ± 39 minutes per patient with ischemia. Forty-four percent of the no heparin group had 47 episodes of ischemia with a mean of 13 ± 13 minutes per episode and a total ischemic time of 58 ± 47 minutes per patient with ischemia. Multiple linear regression analysis to adjust for intergroup differences did not alter the results. Eighty-five percent of all episodes were asymptomatic. Clinical events, such as episodes of chest pain, emergency coronary arteriography, or coronary revascularization, were also similar between groups. Thus the short-term administration of heparin did not alter the incidence or duration of ischemia in patients with unstable angina. The effects of adjuvant anticoagulation may be masked by the concomitant use of aspirin, may require longer administration to reduce ischemia, or may exert benefits by a mechanism other than reducing myocardial ischemia. © 1991.