The purpose of this study was to identify if obesity or obstructive sleep apnea (OSA) influence hypercapnic response (HCR) and hypoxic response (HR) taking into account differences between gender. Data are expressed in % predicted based on reference values of our laboratory ('Respir. Physiol. 113 (1998) 157'). Obese women without OSA (n=117, body mass index (BMI) 43+/-8 kg/m(2)) demonstrated an increased ventilatory (VE) and occlusion pressure (P(0.1)) HCR and HR slope: VE HCR 113 (NS), VE HCR/vital capacity (VC) 126 (P<0.05), P(0.1) HCR 130 (P<0.05), VE HR 136 (P<0.05), VE HR/VC 154 (P<0.001) and P(0.1) HR 210 (P<0.001) % predicted. Obese women with OSA (n=34, BMI 42+/-9 kg/m(2)) presented similar increased values for HCR and even more increased values for HR than obese women without OSA matched for age, height and BMI: VE HR 155 (P=NS), VE HR/VC 205 (P<0.05) and P(0.1) HR 273 (P<0.05) % predicted. In obese men (without or with OSA) HCR and HR values were similar to our reference values.