We studied hypoxic and hypercapnic ventilatory drives in 22 eucapnic obese subjects (14 female and eight male subjects) referred for weight reduction therapy and 23 normal subjects (eight female and 15 male subjects). In the female subjects, both occlusion pressure, currently used as an indicator of ventilatory drive, and ventilatory responses to hypoxia, as well as occlusion pressure response to hypercapnia, were significantly greater in the obese than in the normal subjects; however, no significant differences in these responses between male obese and male normal subjects were observed, except for the hypoxic occlusion pressure response. We also studied disordered breathing during sleep in the obese subjects, and male predominance in abnormal breathing and oxygen desaturation was noted. These results showed that obese female subjects increased their hypoxic and hypercapnic chemosensitivities against their body mass loading, which was not evident in obese male subjects. The relatively depressed chemosensitivities of the latter may be related to disordered breathing and oxygen desaturation during sleep.