A decline is usually observed in the myocardial uptake of thallium-201 in delayed imaging compared with initial imaging. In patients with severe coronary artery disease (CAD), the uptake is sometimes higher in the delayed than in the initial imaging, which is expressed as negative washout rate. To evaluate the diagnostic implications of this negative washout rate the findings of dipyridamole thallium scintigraphy in 582 patients with coronary artery disease were evaluated. The negative washout rate was present in 201 of 582 patients (35%). It had a significant association with high grade coronary artery narrowing of > or = 90%. Sensitivity in detecting patients with this high grade narrowing by negative washout rate was 48%, its specificity was 93%, and its positive predictive value was 94%. Sensitivity to detect individual coronary artery narrowing of > or = 90% did not decrease according to the extent of CAD, with the highest detection in the left anterior descending coronary artery and the lowest in the left circumflex coronary artery. Since patients with high grade coronary narrowing often require coronary intervention, the results of this study suggest the diagnostic importance of negative washout rate in the identification of the particular subset of patients with CAD.