Quadrupedal limb coordination during human walking was recently shown to be upregulated during obstacle stepping. An anticipatory activity of coupled cervico-thoraco-lumbar interneuronal circuits is followed by an appropriate executory activation of leg and arm muscles during task performance. This mechanism was studied in subjects with Parkinson's disease and age-matched controls walking on a treadmill with a randomly approaching obstacle. Spinal reflex (SR) responses, evoked by tibial nerve stimulation during mid-stance, were present in all arm and leg muscles investigated. They were larger before execution of obstacle avoidance compared with normal steps in both subject groups. The performance of obstacle stepping was slightly worse in Parkinson's disease than in control subjects. The anticipatory SR in the arm muscles prior to normal and obstacle steps was larger in Parkinson's disease compared with age-matched subjects, but smaller in the tibialis anterior. The arm and leg muscle activation was stronger during obstacle compared with normal swing but did not differ between Parkinson's disease and age-matched subjects. These observations indicate that quadrupedal limb coordination is basically preserved in Parkinson's disease subjects. Our data are consistent with the proposal that in Parkinson's disease subjects the enhanced anticipatory spinal neuronal activity (reflected in the SR) in the arm muscles is required to achieve an appropriate muscle activation for the automatic control of body equilibrium during the performance of the task. In the tibialis anterior the SR is attenuated presumably because of a stronger voluntary (i.e. cortical) control of leg movements.