The Epworth Sleepiness Scale is used frequently to measure excessive daytime sleepiness in research and clinical settings, although there is limited evidence on test–retest reliability, particularly among sleep clinic populations. The objective of this study was to evaluate the reliability of this instrument among adult patients recruited from a public hospital sleep clinic in Sydney, Australia. English‐speaking participants self‐completed the Epworth Sleepiness Scale on two occasions, at the specialist clinic visit and on the night of diagnostic polysomnography. Of the 108 participants included in the study, the majority were male (64%) and the mean age was 51 years. The median retest interval was 64 days. The primary outcome of test–retest reliability as measured using the intraclass correlation coefficient was 0.73 (95% confidence interval, 0.61–0.82). Despite moderate statistical reliability and a low mean difference of 1.1, Bland‐Altman analysis showed an unacceptably wide distribution of between‐score differences. The 95% limits of agreement were −8.5 to +10.6, and an absolute difference in scores of at least 3 was observed in 60 (56%) of the participants. Our results suggest that the Epworth Sleepiness Scale should not be used in clinical settings to make individual‐level comparisons, such as the effect of therapeutic interventions, or to prioritise access to services.