The benefits of continuous positive airway pressure (CPAP) therapy in patients with the sleep apnea/hypopnea syndrome (SAHS) are poorly documented and patients use CPAP less than physicians recommend. To establish patients' perceptions of benefit from CPAP and to identify determinants of CPAP use, 204 CPAP users completed a questionnaire relating to use of CPAP therapy, sleepiness, and road traffic incident rate before and after CPAP, perceived change in daytime function and nocturnal symptoms with treatment, and problems with CPAP. Variables from these domains of interest were examined, reduced through principal components analysis and correlated to assess associations between these and polysomnographic measures of illness severity. Self-reported CPAP use averaged 5.8±SD 2 h a night. Subjective sleepiness rated by the Epworth sleepiness scale and road traffic incident rate were significantly reduced by CPAP (p<0.0001). A broad range of function and symptom items were highly significantly improved with CPAP (p<0.0001), corroborating the cost to community and industry from SAHS and the preventive value of CPAP. Road traffic incident rate before treatment was correlated with pre-CPAP sleepiness and SAHS severity. Subjective CPAP use correlated with sleepiness before treatment but not with SAHS severity. CPAP mask problems and side effects were not associated with reduced CPAP use, but "nuisance" complaints of awakenings, noise, and sore eyes from CPAP correlated negatively with reported use. Greater reported CPAP use was associated with better resolution of sleepiness and greater improvement in daytime function and nocturnal symptoms.