The aim of the present study was to examine the relationships between the responses to progressive isocapnic hypoxia and hypoxic withdrawal test in patients with obstructive sleep apnoea-hypopnoea syndrome (OSAHS) and to analyse the determinants of carotid body sensitivity in OSAHS. Nineteen consecutive OSAHS patients and 13 healthy subjects were selected. Ventilatory (delta V'I/Sa,O2/BSA) and inspiratory neural drive (delta P0.1/Sa,O2) responses to progressive isocapnic hypoxia were determined. Peripheral chemosensitivity was evaluated by the hypoxic withdrawal test, which measures the decrease in ventilation caused by two breaths of 100% oxygen (%delta V'I). Withdrawal response and ventilatory and inspiratory neural drive responses to hypoxia were lower in OSAHS patients than in control subjects. In patients with OSAHS, %delta V'I correlated significantly with delta V'I/Sa,O2/BSA and with delta P0.1/Sa,O2. On stepwise multiple linear regression analysis, a strong correlation between %delta V'I and delta P0.1/Sa,O2 was found. Moreover, %delta V'I, delta V'I/Sa,O2/BSA and delta P0.1/Sa,O2 were significantly correlated with minimum arterial oxygen saturation and with arousal index. Obstructive sleep apnoea-hypopnoea syndrome patients have a strong relationship between peripheral chemosensitivity and respiratory response to hypoxia, suggesting that hypoxic stimulation of central chemoreceptors is minimally relevant in obstructive sleep apnoea-hypopnoea syndrome. Moreover, sensitivity of the carotid body in patients with obstructive sleep apnoea-hypopnoea syndrome is related to sleep disruption and to nocturnal hypoxia.