The Epworth Sleepiness Scale (ESS) was used to measure the degree of daytime sleepiness in two groups of patients with obstructive sleep apnoea (OSA), before and after treatment with nasal continuous positive airways pressure (CPAP). One group (50 patients) were assessed after 2 months CPAP treatment after which the mean ESS fell from 16.4 [standard error of mean (SEM) 0.52] to 7.0 (SEM 0.56). A second group (25 patients) were assessed after 1 yr of treatment: a similar fall in mean ESS was seen from 15.2 (SEM 1.13) before treatment to 6.0 (SEM 0.72). These results imply that the ESS can be used clinically to demonstrate the response of daytime sleepiness in OSA to treatment with CPAP, and that the fall in ESS seen after 2 months is sustained after 1 yr of treatment. It is also possible that this approach could be used to monitor the progress of treatment with CPAP.