Peripheral Arterial Disease (PAD) commonly manifests symptomatically as intermittent claudication (walking-induced muscle pain), primarily in the calf muscles. Claudication is likely caused by insufficient perfusion, but the specific perfusion dynamics are not well understood. In healthy humans, passively induced elevations in local tissue temperature acutely stimulate vasodilation in both skin and muscle microvasculature (1) and increased perfusion in conduit arteries (2). Chronically, increased perfusion and the related blood flow and shear stress profile are important for vascular health; increased antegrade shear stimulates beneficial adaptations (3). The aim of this study was to determine if lower-limb heating in patients with PAD could increase lower-limb inflow and muscle perfusion. Eight patients with PAD (6 male, age 69 ± 5 y) and nine controls free from PAD (8 male, age 71 ± 6 y) underwent hot-water immersion (30 min immersed to the waist in water at 42-43°C). Popliteal artery perfusion was assessed before, during the last 3 min and 30 min after immersion, using high-resolution ultrasound. Haemodynamics of the medial gastrocnemius muscle were monitored throughout using near-infrared spectroscopy. Popliteal artery blood flow at baseline was lower in PAD (28 ± 13 mL·min-1) than in controls (50 ± 11 mL·min-1); it increased approximately three-fold in PAD and two-fold in controls during water immersion (PAD: +71 ± 49 mL·min-1, p < 0.01; controls: +92 ± 59 mL·min-1, p < 0.01), and remained elevated at 30 min after immersion (PAD: +20 ± 20 mL·min-1, p = 0.04; controls: +31 ± 31 mL·min-1, p = 0.02). At the muscle, oxyhaemoglobin volume increased to a lesser extent in PAD than in controls (+261 ± 119 uM vs. +502 ± 246 uM; interaction: p = 0.01), while deoxyhaemoglobin volume was not significantly altered in either group (PAD: +10 ± 42 uM, controls: +13 ± 57 uM). Overall, the increased perfusion of the muscle during hot-water immersion was evident by an increase in total haemoglobin volume for both groups, more so in the controls (PAD: + 271 ± 94 uM, p = 0.01; controls: +524 ± 270 uM; interaction: p = 0.01). These perfusion changes were reflected in an overall increase in tissue oxygenation index of the medial gastrocnemius muscle of ~7.4 ± 8.1% in PAD (p = 0.02) and ~7.3 ± 9.5% in controls (p = 0.03). A single bout of lower-limb heating by hot-water immersion significantly increased popliteal artery inflow and acutely improved muscle perfusion in the lower limbs of patients with PAD, although the magnitude is somewhat lower to that seen in healthy controls. Lower-limb heating may have therapeutic potential for PAD patients as has previously been suggested (4); therefore the functional and long-term adaptive effects of repeated lower-limb heating in atherosclerotic arteries should be explored.