© 2016 by the American Society of Nephrology. Background and objectives Arteriovenous fistulamaturation requires an increase in the diameter and blood flow of the feeding artery and the draining vein after its creation. The structural properties of the native vessels may affect the magnitude of these changes. We hypothesized that an increase in the collagen content of the vascular media (medial fibrosis) preoperatively would impair vascular dilation and thereby, limit the postoperative increase in arteriovenous fistula diameter and blood flow and clinical arteriovenous fistula maturation. Design, setting, participants,&measurementsWeenrolled 125 patients undergoing arteriovenous fistula creation betweenOctober of 2008 andApril of 2012 and followed themprospectively.Any consenting subjectwas eligible. Arterial and venous specimens were sampled during arteriovenous fistula surgery.Masson’s trichrome-stained samples were used to quantify medial fibrosis. Arteriovenous fistula diameter and blood flow were quantified using 6-week postoperative ultrasound. Clinical arteriovenous fistula maturation was assessed using a predefined protocol. The association of preexisting vascular medial fibrosis with arteriovenous fistula outcomes was evaluated after controlling for baseline demographics, comorbidities, and the preoperative venous diameter. Results Themeanmedial fibrosiswas 69%±14%in the arteries and 63%±12%in the veins.Arterial medial fibrosis was associated with greater increases in arteriovenous fistula diameter (Δdiameter =0.58 mm; 95% confidence interval [95% CI], 0.27 to 0.89mm; P<0.001) and arteriovenous fistula blood flow (Δblood flow=85ml/min; 95% CI, 19 to 150 ml/min; P=0.01) and a lower risk of clinical arteriovenous fistula nonmaturation (odds ratio, 0.71; 95%CI, 0.51 to 0.99; P=0.04), all per 10%absolute difference in medial fibrosis. In contrast, venous medial fibrosis was not associated with the postoperative arteriovenous fistula diameter, blood flow, or clinical maturation. Conclusions Preoperative arterial medial fibrosis was associated with greater arteriovenous fistula diameter and blood flow and a lower risk of clinical arteriovenous fistula nonmaturation. This unexpected observation suggests that medial fibrosis promotes arteriovenous fistula development by yet undefined mechanisms or alternatively, that a third factor promotes both medial fibrosis and arteriovenous fistula maturation.