Exacerbation of respiratory failure in Parkinson's disease could be the result of impaired perception of hypoxia. We assessed chemosensitivity to hypoxia and hypercapnia and perception of dyspnoea on the Borg scale in 25 patients (Hoehn and Yahr stage 2-3) and 11 controls. Chemosensitivity to hypoxia, but not that in response to hypercapnia, was lower in patients than in controls (0.196 [SE 0.030] vs 0.525 [0.360]; p=0.012); the mean Borg score was lower in patients than in controls under hypoxic conditions (2.9 [SD 1.4] vs 4.8 [2.1]; p=0.0015). Thus, even at an early stage of disease, patients with Parkinson's disease had a subnormal hypoxic response accompanied by blunted perception of dyspnoea.