Ultrasonography is the primary tool for preoperative analysis of vein morphology for fistula creation in patients with end-stage renal disease. This study examines the effect of environmental temperature on the superficial vein size. Superficial veins of thirteen healthy volunteers were marked at three sites: cephalic vein in left lateral arm near cubital fossa, cephalic vein in left forearm at wrist, and basilic vein in left medial arm near cubital fossa. Mean diameters were recorded using ultrasound probe at 26°C and 43°C. Body temperature was increased using a Bair Hugger blanket. Mean values from the two temperatures were analyzed using paired sample t-test. All three superficial vein sites displayed statistically significant increase in diameter when the temperature was increased from 26°C to 43°C. Paired t-test showed p values of 0.001 for cephalic vein at wrist, 0.01 for cephalic vein near cubital fossa, and 0.01 for basilic vein near cubital fossa. This study proved that environmental temperature exerts a statistically significant effect on vein size measured by ultrasound during preoperative assessment for vascular access. Not to the extent of 43°C, the authors would recommend setting the room temperature higher during ultrasound vascular assessment to avoid underestimating the superficial vein size.