Individuals with myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS) have little stamina and endurance, and pose a challenge for nursing professionals. The Energy Envelope Theory, which posits that maintaining expended energy levels consistent with available energy levels may reduce the frequency and severity of symptoms, is particularly useful when working with clients with ME/CFS. Anecdotal support from the client community for this theory supports its use as a management tool for ME/CFS, but little formal research has been done in this area. In this study, a daily energy quotient was established by dividing the expended energy level by the perceived energy level and multiplying by 100. It was predicted that those participants who expended energy beyond their level of perceived energy would have more severe fatigue and symptoms and lower levels of physical and mental functioning. Findings are congruent with the Energy Envelope Theory as they indicated that the daily energy quotient was related to several indices of functioning including depression, anxiety, fatigue, pain, quality of life, and disability. The overall results provide support for a strategy health care professionals can use when working with clients with ME/CFS.