Collateral circulation of the hand and digits was evaluated in 20 healthy male volunteers using a modified Allen's test and photoelectric plethysmography. A borderline (7-14 sec) Allen's test was present in 12.5% and 2.5% required more than 15 seconds for thenar flush. Photoplethysmography was then used to compare the relative contributions of ulnar artery flow to total arterial flow into the hand. This ratio, (ulnar/total) was termed the flow index (FI). Average FI for the first digit (thumb) was 0.59, and for the second digit (index) was 0.64. The FI difference between the first digit and that of the third, fourth and fifth digits was statistically significant (p less than 0.01). The FI difference between the second digit and the third, fourth and fifth digits was also statistically significant (p less than 0.05). These results imply that Allen's test may be misleading, indicating adequate collateral circulation when in fact collateral blood flow to the digits may be poor or nonexistent.