Mountain endurance running has increased in popularity in recent years. Thus the aim of the present study was to determine if maximal oxygen uptake (VO₂max) and energy cost of running (C_r) measured during level and uphill running are associated. Ten high level male endurance mountain runners performed three maximal oxygen uptake tests at three slope conditions (0, 12.5 and 25%). Metabolic data, step frequency (SF) and step length (SL) were recorded. No significant differences were found in VO₂max (63.29 (±3.84), 63.97 (±3.54) and 63.70 (±3.58) mlO₂/kg^-1/min^-1) or associated metabolic data at 0, 12.5 and 25% slope respectively. High intra-individual correlations were found between metabolic data measured in the three conditions. The energy cost of running was significantly different between slopes (0.192 (±0.01), 0.350 (±0.029) and 0.516 (±0.035) mlO₂/kg^-1/min^-1, p < 0.01), 0, 12.5 and 25% respectively. However, Cr_0% was not correlated with either Cr_25% or Cr_12.5% (r_s = 0.09 and r_s = 0.10), in contrast, Cr_25% and Cr_12.5% were correlated (r_s = 0.78). Step length was positively correlated with speed under the three slope conditions. Step frequency was significantly lower at 25 compared to 12.5 and 0% slope. We found that the maximum aerobic power did not differ between level and graded treadmill tests. However, the increase in Cr on the inclined versus level conditions varied between subjects. None of the measured anthropometric or kinematic variables could explain the higher increase in Cr of some subjects when running uphill. Thus, a short graded (5min at 12.5%) running test should be performed at a submaximal velocity (around 40% of level vVO₂max) to enhance understanding of an endurance runner’s uphill capability.