In healthy subjects, comfortable walking minimizes the energy cost (E (c)) of locomotion. In Parkinson's disease (PD) patients walking is slower than in healthy subjects: this may increase E (c). Our aims were to analyze gait and E (c) in PD patients during walking, particularly at self-selected speed, and the possible pathological, mechanical, and cardiorespiratory limitations. Fourteen mild-to-moderate PD and 14 control subjects were enrolled. Subjects underwent 5-min walking tests at two speeds: self-selected and as-fast-as-possible speeds. Cardiopulmonary and gait parameters (heart rate, ventilation, gas exchanges, step count) were recorded. Velocity was reduced in PD compared to control subjects at both speeds (P < 0.05), and PD patients had shorter strides (P < 0.05) at both speeds and reduced cadence (P = 0.01) at fastest speed. No significant difference was found in E (c) at self-selected (0.12 ± 0.04 versus 0.11 ± 0.02 mLO(2) kg(-1) m(-1) in PD and control subjects, respectively) and maximal (0.14 ± 0.03 versus 0.15 ± 0.02 mLO(2) kg(-1) m(-1) in PD and control subjects, respectively) speed. However, the E (c) increment from self-selected to fastest velocity was significantly lower (P = 0.02) in PD patients. PD patients failed to walk at a self-selected speed, which minimizes the E (c). This could be mainly due to the inability to develop a wider stride. Cardiorespiratory adaptation was not affected, except for the possible reduced cardiac adaptation observed in some (28%) cases. Presumably, rehabilitation procedures that improve flexibility and step length may help maintain walking ability.