Coronary blood flow was quantified using the thrombosis in myocardial infarction (TIMI) frame-count method. This measurement has been significantly correlated with flow velocity measured invasively by use of a Doppler flow wire. Coronary artery ectasia or aneurysm (CEA) is thought to be present in patients with a slow blood flow. In this study, we aimed to assess the relationship between the ectasia size or ectasia ratio and TIMI frame count in patients with CEA. The study population included 58 patients with isolated CEA of the right coronary artery. In patients with CEA, an ectasia ratio was calculated as diameter of the ectatic segment/diameter of the adjacent normal segment. According to the ectasia ratio, ectatic vessels were divided into two groups: ectasias with a 1.5- to 2.0-fold increase (group A) and more than 2.0-fold increase (group B) in normal vessel diameter. Patients with a significant stenotic lesion (> 50%) in the ectatic vessel were excluded. The control group was formed from a matched population of 35 patients with angiographically proven normal coronary arteries. Characteristics of the ectasia and control groups are similar. The TIMI frame counts for the right coronary artery (RCA) were significantly higher in the ectasia group as compared with the control group (43 +/- 12 vs 23 +/- 8, P < 0.001). The ectasia group had 38 patients in group A and 20 patients in group B. The TIMI frame counts were significantly higher in group B than in group A (43 +/- 10 vs 51 +/- 15, P < 0.05). The TIMI frame count of the RCA showed a significant correlation with the ectasia ratio and the maximum diameter of the ectatic segment (r = 0.578, P < 0.001 and r = 0.435, P < 0.001, respectively). Our data suggest that TIMI frame count measurement depends on the ectasia size or ectasia ratio, and an increased ectasia ratio is markedly associated with decreased TIMI frame counts in patients with CEA.