This study has investigated the role of average maximum inspiratory effort in excessive daytime sleepiness in patients with obstructive sleep apnoea syndrome (OSAS) and upper airway resistance syndrome (UARS). Fifteen patients diagnosed with UARS and 32 patients with OSAS, with >5.5 h total sleep time (TST) during 8 h of nocturnal polygraphic recordings, were included in the study. Demographical data, polysomnographical data and data about daytime sleepiness, including Epworth sleepiness scale (ESS) and multiple sleep latency test (MSLT), were evaluated. In order to compute the average maximum inspiratory effort from oesophageal pressure (Poes) measurements, maximum Poes was obtained from 20 representative obstructive respiratory events (obstructive apnoeas, hypopnoeas or flow limitations) for each sleep stage in both supine and side positions. From Poes measurements during sleep, the increase in Poes (deltaPoes) during respiratory events was also calculated. The average maximum Poes, deltaPoes, respiratory disturbance index (RDI) and arousal index were significantly correlated with ESS in OSAS patients. In patients with UARS, the only significant correlation was obtained between average maximum Poes and ESS. The MSLT score did not show any significant correlation with arousal index, number of stage variations, RDI, average Poes, deltaPoes, minimum oxygen saturation (Sa,O2) and percentage of TST with an Sa,O2 <90% in both UARS and OSAS patients. The results of multiple regression analysis showed that average maximum Poes correlates best with the variance in ESS for OSAS patients. In conclusion, the data from this study indicate the possible important role of average inspiratory effort in determining subjective sleepiness in both obstructive sleep apnoea syndrome and upper airway resistance syndrome patients.