Purpose: To retrospectively determine whether postoperative ultrasonography (US) of clinically immature dialysis fistulas can be used to identify potential anatomic origins of immaturity and whether correction of immature fistulas promotes fistula maturation. Materials and Methods: Institutional review board approval was obtained for this retrospective study, with waiver of informed consent. Postoperative US was performed in 95 consecutive subjects (52 men, 43 women; mean age, 58 years) with clinically immature fistulas. Fistulas were deemed sonographically mature if they had a diameter of at least 4 mm, an access flow of at least 500 mL/min, and a depth of no more than 5 mm from the skin surface. Fistulas were deemed clinically mature if they could be used for dialysis for at least 1 month. The proportion of subjects in different US subgroups with fistulas that matured during follow-up was compared with χ2 analysis. Results: Sixty-seven clinically immature fistulas were deemed sonographically immature. One or more remediable anatomic problems were detected with US in 60 subjects with sonographically immature fistulas; these problems included focal stenosis in 23, accessory veins in 34, and excessively deep veins in 19. Multiple abnormalities were present in 13 subjects. Of 58 subjects with sonographically immature fistulas and known clinical outcomes, 32 underwent an intervention. In those subjects who did not undergo a salvage procedure, only eight fistulas were usable for dialysis. Among those who underwent a salvage procedure, 25 fistulas were subsequently usable for dialysis (P < .001). Conclusion: Clinically immature fistulas frequently have one or more potentially remediable problems seen at postoperative US. Problem-specific salvage procedures increase the proportion of fistulas that are usable for dialysis. © RSNA, 2007.