Two methods of analysis for perfusion myocardial studies with thallium are compared: the conventional visual analysis, and a quantitative method which shows results as circumferential profiles. Three hundred and ninety myocardial segments in 65 patients were studied. Visual analysis showed abnormalities in 44/65 (68%) patients, the quantitative method did it in 53/65 (81%). When localization and/or extension discrepancy between the two methods was found, angiography was always concordant with circumferential profiles findings. Total agreement between the two methods was present in 20/65 (31%) patients. The quantitative method is more sensitive than the visual analysis. It is also more precise in defining localization and extension of thallium defects.