There are limited published data concerning the frequency and relative intensity of abdominal activity on 99Tcm-methoxyisobutyl isonitrile (99Tcm-MIBI) myocardial perfusion scans and its effect on interpretation. We undertook a blinded prospective study to evaluate (1) the frequency and intensity of abdominal activity on single photon emission tomography (SPET) scans, (2) its effect on separate evaluation of rest and stress SPET images, and (3) its effect on clinical interpretation. Patients undergoing one-day rest-stress 99Tcm-MIBI scans were randomized to receive 99Tcm-MIBI obtained from one of two radiopharmacies. The rest plus exercise or rest plus intravenous dipyridamole scans of 303 patients were scored separately by three physicians for (1) intensity of abdominal activity and (2) its effect on scan evaluation. Nuclear reports generated independently of the blinded evaluation were reviewed to assess the effect of abdominal activity on clinical interpretation. There were no statistical differences between pharmacies. Abdominal activity was uncommon on the exercise but common on the rest and dipyridamole scans. The exercise scans differed from the rest and dipyridamole scans in the subgroups: intensity of abdominal activity equal to myocardium, and greater than myocardium (P < 0.001). There was no difference between the rest and dipyridamole scans. The effect on evaluation was moderate in 5% of the exercise, 46% of the dipyridamole and 37% of the rest scans, and severe in 1% of the exercise, 3% of the dipyridamole and 12% of the rest scans. Rest differed from exercise (P < 0.001) and from dipyridamole (P < 0.05). There was no difference between the dipyridamole and exercise scans. Based on the clinical reports, abdominal activity was a limitation to scan interpretation for 20 patients; in 5, the inferior wall could not be evaluated. Although abdominal activity was frequently observed on both the dipyridamole and rest scans, it was a limitation to clinical interpretation in a small fraction of the patients.