Background and objectives: Failure to mature (primary failure) of new fistulas remains a major obstacle to increasing the proportion of dialysis patients with fistulas. This failure rate is higher in women than in men, higher in older than in younger patients, and higher in forearm than in upper arm fistulas. These disparities in the frequency of failure to mature may be due in part to marginal vessels in the high-risk groups and should be reduced by routine preoperative vascular mapping. Design, setting, participants, & measurements: A prospective, computerized database was queried retrospectively to evaluate the frequency of primary fistula failure in 205 hemodialysis patients for whom preoperative mapping was obtained. The association between clinical characteristics and risk for primary fistula failure was analyzed by univariate and multiple variable regression analysis. Results: The overall primary fistula failure rate was 40% (82 of 205 patients). On multiple variable logistic regression, three clinical factors were associated with an increased risk for failure to mature among patients who underwent preoperative vascular mapping: Female gender, age ≥65 yr, and forearm location. The primary fistula failure rate varied from 22% in younger men with an upper arm fistula to 78% in older women with a forearm fistula. Dynamic preoperative vascular measurements (change in peak systolic velocity and resistive index after tight fist clenching) did not differ between patients with mature and immature forearm fistulas. Conclusion: Disparities in fistula maturation persist despite the use of routine preoperative vascular mapping. Copyright © 2008 by the American Society of Nephrology.