INTRODUCTION: Excessive daytime sleepiness (EDS) and fatigue occur in high percentages in the general population. They are common complaints in primary care and in specialty medicine. Although they may represent distinct or overlapping phenomena, the general medical literature does not normally distinguish between EDS and fatigue. Despite their prevalence, both EDS and fatigue are identified and treated in a relatively small proportion of those affected. The similarity of EDS and fatigue may create diagnostic ambiguity and thereby contribute to under-identification and under-treatment. Fatigue, in particular, is thought to be difficult to manage when it is identified. METHODS: The literature was searched for reviews, meta-analysis and similar levels of papers focused on EDS or fatigue. RESULTS: EDS and fatigue are operationalized in ways that contribute to blurring rather than to distinguishing between them. Existing measures of both EDS and fatigue may also contribute to their misidentification. Effective treatments for both symptoms have been established. Behavioral interventions are effective and underutilized. DISCUSSION: We suggest more precise operationalization of EDS and fatigue, leading to a refinement of existing measures or development of new tools, a structured interview with fatigue and EDS sections in the clinical setting, and more consideration for behavioral interventions. Copyright 2003 Elsevier Science Inc.