To evaluate the association of heart rate (HR) response with abnormal scan and/or left ventricular (LV) function in patients undergoing adenosine myocardial perfusion imaging, we prospectively studied 164 consecutive patients who underwent a standard adenosine stress test (without exercise) and myocardial perfusion imaging (MPI) using technetium-99m sestamibi radioisotope. Change in HR was calculated by subtracting HR at rest from peak HR. The percentage change in HR was calculated. All patients underwent stress and resting single photon emission computed tomography (SPECT) imaging. Left ventricular ejection fraction (EF) was calculated using gated SPECT. Mean age was 54 ± 11.7 years and 126 of the patients (72%) were men. We divided the patients into 2 groups: group 1(42 patients, 25%) had normal scans and group 2(122 patients, 74.3%) had abnormal scans; abnormal scans were defined as presence of either fixed defects, reversible defects, or both. Average HR increased by 35 beats/min in the normal scan group compared with 23 beats/min in the abnormal scan group (p=0.002). Sixty four (64) patients (39%) had reduced EF (<45%). This group had an average HR and percentage HR increase of 23 beats/min (27%) compared with an increase of 35 beats/min (38%) in patients with normal EF (p=0.002 and p=0.02, respectively). Thus, a diminished HR response had a significant association with both an abnormal scan and reduced EF on adenosine MPI.