Some patients with obstructive sleep apnoea syndrome (OSAS; respiratory distress index (RDI) of >5 events.h(-1)) experience residual excessive daytime subjective sleepiness (Epworth Sleepiness Scale (ESS) score of >10), despite adequate use of continuous positive airway pressure (CPAP) therapy. The aim of the present study was to identify clinical and polysomnographic predictors of this sleepiness. Clinical and polysomnographic variables and ESS score were evaluated in 208 OSAS patients with an ESS score of >10 before (initial assessment) and after > or =6 months of adequate (> or =4 h.day(-1)) CPAP use. Following CPAP treatment, 114 (55%) patients showed an abnormal ESS score (>10; CPAP nonresponders), whereas 94 (45%) showed a normal ESS score (<11; CPAP responders). Of the CPAP responders, none had a history of depression, whereas the converse was true for 38.8% of CPAP nonresponders. In addition, multivariate logistic regression analysis revealed that the independent predictors of residual excessive daytime sleepiness following CPAP therapy were a history of diabetes and heart disease, and a higher ESS score and lower RDI on initial assessment. In conclusion, predictors of residual excessive sleepiness in adequately CPAP-treated OSAS were a history of depression, diabetes and heart disease, and a higher ESS score and lower RDI on initial assessment.