<h3>Research Objectives</h3> To capture symptom experiences of persons living with ME/CFS to increase clinicians' understanding of this complex illness. <h3>Design</h3> Case Series. <h3>Setting</h3> Multi-institutional collaboration. <h3>Participants</h3> A convenience sample of 12 persons with mild (n=16.7%), moderate (n=58.3%), and severe (n=25.0%) ME/CFS. Participant age: mean = 55.4 (SD = 9.7), range = 39-71. Ten participants identified as female, one as male, and one as transgender. Participants self-identified as Black or African American (n=16.7%), Asian (n=8.3%), White (n=75.0%), and Hispanic/Latinx (n=8.3%). <h3>Interventions</h3> None. <h3>Main Outcome Measures</h3> Not applicable. <h3>Results</h3> Language to describe symptoms, collected from semi-structured interviews, was categorized into domains recognized by the National Academy of Medicine [2]: post-exertional malaise (PEM), fatigue, cognitive impairment, unrefreshing sleep, pain, orthostatic intolerance/dysautonomia. Content analysis revealed a lexicon used by persons with ME/CFS to describe symptoms. Examples include "brain fog" (n=7/12) to depict cognitive impairment; "energy envelope" (n= 4/12) or "battery" (n=2/12) to explain the consequences of exceeding physical or mental activity thresholds causing PEM, the hallmark symptom of ME/CFS; "tired yet wired" (n=2/12) to describe unrefreshing sleep; and "dizzy" (n=6/12), "woozy" (n=1/12), and "swimmy" (n=1/12) to characterize orthostatic intolerance. Findings demonstrate similar use of terms and phrases to describe symptoms and experiences. <h3>Conclusions</h3> ME/CFS is a poorly understood, disabling illness. This study builds on prior work to define ME/CFS by analyzing lived experiences of those with this condition [3,4,5,6]. Awareness of language used by persons with ME/CFS to describe symptomatology may facilitate patient-clinician interactions and enable clinicians to better manage patient symptoms. ME/CFS is a diagnosis of exclusion and persons with ME/CFS often receive different diagnoses, perhaps due to complexity of symptoms, language used to describe them, or a general lack of understanding the illness. Delayed or misdiagnosis may result in triggering PEM and worsening symptoms. Further comprehension of language utilized by patients may help rehabilitation clinicians working with patients avoid PEM due to unnecessary overexertion. These findings may also be explored for persons with Long COVID due to common, overlapping symptoms. <h3>Author(s) Disclosures</h3> Nothing to disclose.