The modified Allen's test (A.T.) is used to prove the adequacy of the blood supply through the ulnar artery to the hand. The test is considered normal if, after compression of both the ulnar and radial arteries followed by repeated clenching of the fingers to squeeze out the blood, the normal or a slightly more pronounced red color of the palm of the hands returns within < 10 sec after release of only the ulnar artery. It is important to perform the test whenever intravascular access to the radial artery is planned. The objective of this study was to determine the percentage of patients presenting a clearly positive (< 5 sec), a moderately positive (between 5-9 sec), or a negative A.T. (10 sec and more), We performed an A.T. on 1,000 consecutive and different patients undergoing cardiac catheterization, on the right wrist, two times, each time by a different examinator trained in this test. An A.T. was performed on 1,000 patients:640 men, 360 women, mean age 62.3 yr (range 28-90 yr). In our population of patients, the relative percentages of a clearly positive (< 5 sec), moderately positive (between 5-9 sec), or negative modified Allen's test (10 sec and more) is 49%, 24%, and 27%, respectively.