Measuring mechanical efficiency (ME) is potentially useful to assess motor performance in individuals with physical disabilities. The purpose of this study was to determine the accuracy of predicting ME from heart rate (HR) during a self-paced stair-climbing test in children with a range of motor abilities. The participants were 12 normally developed children (ND) and 24 with cerebral palsy (CP), ranging in age from 5 to 15 years (mean: 8 years). Five were at level II, 11 at level III and 8 at level IV according to the gross motor function classification system. ME was calculated as the ratio of external work to O₂ uptake (V˙O₂ ml/min) measured or predicted from HR. The absolute values of V˙O₂ and HR during stair-climbing were not significantly correlated. However, the correlation between values above resting (dV˙O₂ and dHR) was significant (r=0.61). Furthermore, when including body weight as a second variable the prediction of dV˙O₂ was significantly improved (r=0.85). This resulted in a high correlation (r=0.96) between measured and predicted net ME (ME_net). Predicted ME_net for 25 stair-climbing tests repeated after an average of 6 months resulted in an r-value of 0.92 with predicted ME_net of the first test. This study demonstrates that ME_net during stair-climbing can be predicted in children with a broad range of motor abilities from dHR and may be a simple tool to help define developmental stages or evaluating intervention efficacy.