Severity of negative esophageal pressure (Pes) and apnea hypopnea index (AHI) were investigated in 34 patients with obstructive sleep apnea-hypopnea syndrome (OSAHS). The OSAHS patients were diagnostically classified as having obstructive sleep apnea syndrome (OSAS) or upper airway resistance syndrome (UARS). Diagnosis of OSAS was based on an AHI of more than 5, and that of UARS on an AHI of less than 5, EEG arousals which were associated with apnea, hypopnea and/or respiratory effort occurring more than 10 times per hour, and daytime sleepiness. Negative Pes was represented by the greatest peak (NPes Max) and the number of increased (more than 13.5 cmH2O) episodes per hour (NPesI13.5). There was no significant correlation between the AHI and Pes indices, but NPes Max and NPesI13.5 showed significant correlation (p50; 8 cases), moderate OSAS (50>AHI>15; 10 cases), mild OSAS (15>AHI>5; 9 cases) and UARS (7 cases) groups. We conclude that AHI does not reflect the severity of the increase in negative Pes, which is an important aspect of the pathophysiology of OSAHS. Assessment of OSAHS based on AHI alone may therefore underestimate the risk of increased negative Pes in cases with reduced AHI.