Background: The apnea-hypopnea index (AHI) and the mean apnea-hypopnea duration (MAD) are used to measure the severity of the symptoms of obstructive sleep apnea (OSA). The aim of this study was to compare the use of the MAD with the AHI as indicators of clinical and demographic parameters, blood oxygenation, and sleep parameters in patients diagnosed with OSA by polysomnography (PSG). Material/Methods: A retrospective study included 511 patients with OSA diagnosed by PSG and who had the AHI and the MAD measured according to the guidelines from the American Academy of Sleep Medicine (AASM). The patients were divided into two groups: patients with a short MAD and with a long MAD, according to median duration, and using the inter-quartile range (IQR), as the data were not normally distributed. Clinical and demographic parameters were recorded. Pulse oximetry was used to measure blood oxygen saturation during sleep, sleep structure was recorded, and the Epworth Sleepiness Scale (ESS) questionnaire was used to measure daytime sleepiness. Results: In all 511 patients with OSA, the MAD was significantly, but weakly, correlated with the AHI (r=0.17, P<0.01), but showed no significant associations with patient age (r=0.08, P=0.06), body weight (r=0.014, P=0.75), and height (r=0.06, P=0.16). Patients with a long MAD or severe OSA (n=260) had significantly worse blood oxygen levels and sleep parameters. Conclusions: For patients with severe OSA, this study showed that the MAD was a useful indicator of blood oxygenation and sleep parameters.