Vertical kilometer (VK) races, in which runners gain 1000 m of elevation in <5000 m of distance, are becoming popular. However, few studies on steep uphill running (>25°) exist. Previously, we determined that ~30° is the optimal angle for uphill running, costing the least amount of metabolic energy for a specific vertical velocity. To inform the training and strategy of VK racers, we quantified the metabolic cost of walking and running at various velocities up a 30° incline.At 30°, 11 experienced runners (7 M, 4 F, 30.8 ± 7.9 years, 1.71 ± 0.08 m, 66.7 ± 9.4 kg) walked and ran for 5-min trials with 5-min rest between. Starting at 0.3 ms, we increased treadmill velocity by 0.1 ms for each trial until subjects could not maintain the set velocity. We measured oxygen uptake (ml O kg min) and metabolic power (W kg = metabolic energy per unit time per unit body mass) and calculated metabolic costs of walking (C ) and running (C ) per unit distance (J kg m).Oxygen uptake and metabolic power increased linearly with velocity. Between 0.3 and 0.7 ms, C  < C . At 0.8 ms there was no difference and extrapolation suggests that at faster velocities, running likely costs less than walking.On a 30° incline, metabolic power increases linearly with velocity. At speeds slower than 0.7 ms, walking requires less metabolic power than running (W kg) suggesting most VK racers should walk rather than run.