OBJECTIVE The energy envelope postulates that patients with Myalgic Encephalomyelitis/chronic fatigue syndrome (ME/CFS) will improve functioning when maintaining expended energy levels at the same level as available energy level. METHODS Estimated weekly Energy Quotients were established by dividing expended energy level by perceived energy level and multiplying by 100. Two groups of patients were identified following participation in a non-pharmacologic intervention trial. Some were able to keep expended energy close to available energy and others were not successful at this task. RESULTS Those who were able to stay within their energy envelope had significant improvements in physical functioning and fatigue severity. CONCLUSION Findings suggest that helping patients with ME/CFS maintain appropriate energy expenditures in coordination with available energy reserves can help improve functioning over time. PRACTICE IMPLICATIONS Health care professionals that treat patients with ME/CFS might incorporate strategies that help patients self-monitor and self-regulate energy expenditures.