Objective. To optimize forearm radiocephalic fistula success, many programs recommend a minimal cephalic vein diameter of 0.25 cm or greater on preoperative sonographic mapping. It is not established, however, whether a vein diameter before or after application of a tourniquet should be used in determining suitability for creation of a forearm fistula. Methods. Before forearm radiocephalic fistula placement, preoperative sonographic mapping measured the cephalic vein diameter before and after application of a tourniquet. The patients fell into 2 groups: those with a pretourniquet vein diameter of 0.25 cm or greater (group 1) and those with a pretourniquet vein diameter of less than 0.25 cm that increased to 0.25 cm or greater after application of the tourniquet (group 2). The adequacy of each fistula for dialysis was determined clinically. Results. Among 73 radiocephalic fistulas with known clinical outcomes, 28 were in group 1, and 45 were in group 2, on the basis of sonography. Fistula success rates were similar in group 1 patients (11 [39%] of 28) and group 2 patients (15 [33%] of 45) (P = .624, Fisher exact test). Inclusion of group 2 patients increased the number of patients recommended for placement of forearm fistulas and increased the total number of usable forearm fistulas from 11 to 26. The overall success rate of forearm fistulas was lower in women (19% versus 50%; P = .015, Fisher exact test). Conclusions. The use of a venous tourniquet increases the number of patients eligible for forearm fistulas without decreasing the adequacy rate of these fistulas. Therefore, a tourniquet should routinely be used in patients with small cephalic veins on pretourniquet evaluation. © 2006 by the American Institute of Ultrasound in Medicine.