More than half of people diagnosed with fibromyalgia fit the bill for chronic fatigue syndrome, too. Are they really just the same disease? Fibromyalgia and chronic fatigue syndrome (CFS) are both illnesses characterized by extreme amounts of fatigue. In fact, the conditions seem to be so intertwined that the medical community continues to debate whether fibromyalgia fatigue is simply a different expression of the same disorder that causes CFS. Statistically, fibromyalgia fatigue occurs in more Americans than chronic fatigue syndrome. The U.S. Centers for Disease Control and Prevention believes there are about 5 million people in the United States with fibromyalgia, compared with a little over 1 million people with chronic fatigue syndrome. Still, research has found that the line between fibromyalgia fatigue and chronic fatigue syndrome is a very thin one. The Arthritis Foundation estimates that 50 to 70 percent of people with fibromyalgia also fit the criteria of chronic fatigue syndrome.