The paper “Fear of movement and avoidance behaviour toward physical activity in chronic-fatigue syndrome and fibromyalgia: state of the art and implications for clinical practice”  is interesting but problematic. We agree that mild exercise is good but over exercise can cause relapse. Myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS) and fibromyalgia patients suffer from relapse when is subjected to over exercise. We at the ME/CFS Center at Mount Sinai School of Medicine, New York are investigating the post-exertional malaise seen in (ME/CFS). This is one of the criteria in the Canadian Consensus of diagnosis of ME/CFS.  The notion that ME/CFS is caused by poor conditioning is no longer widely accepted. It is thought that it is an immune system dysfunction, which after onset can be provoked by stress, over exercise or overwork . Indeed, there can be depression, but this depression is secondary to the disease much like any other chronic disease. Cognitive behavioral therapy may be helpful to reverse the secondary depression but one must understand that this depression is indeed secondary and the initial disease entity is in the immune system and must be identified and treated.