In the United Kingdom, CFS covers a number of conditions, including myalgic encephalopathy (ME) also known as post viral fatigue syndrome (PVFS). The latter is characterised by fatigability and slow recovery following minimal exertion, marked fluctuations in the severity of symptoms throughout the day, impaired circulation and the involvement of the CNS. Most patients who fulfill the British criteria for CFS appear to have disorders other than ME including undiagnosed hypothyroidism, masked depression and problems related to lifestyle and nutrition (e.g. deficiencies of vitamins D and B12).

Despite the fact that the Department of Health now accepts ME/CFS as a genuine medical condition, diagnosis can still pose a problem because ME/CFS symptoms are similar to those present in a number of other medical conditions. In addition, there are no examination findings which can confirm the diagnosis. This means there has to be a process of elimination (the exclusion of other conditions) before a diagnosis of ME/CFS can be made. Commonly, a previously fit and active person finds their illness triggered by an infection. Less common triggers include toxins, organophosphate pesticides, vaccinations, major trauma or stress (e.g. a road traffic accident), pregnancy and surgical operations. In some cases there is no obvious precipitating event and the person reports a gradual decline in health over a period of months or even years.

The predominant symptom of ME/CFS is usually severe fatigue and malaise following mental or physical activity, the full extent of this exhaustion often becoming apparent only 24 to 48 hours after the activity (assuming, of course, the person was not already in a 'recovery period' from a previous activity)

The other main symptoms can be categorised as:

Muscle symptoms- include exercise intolerance and post-exertional malaise (i.e. feeling shattered the day after undue physical activity), pain/myalgia (present in around 75% of people) and fasciculations (visible twitching of the muscles which sometimes includes blepharospasm/eyelid twitching).

Brain and Central Nervous System symptoms- include cognitive dysfunction (problems with short-term memory, concentration and maintaining attention), clumsiness, disequilibrium likened to 'walking on rubber', and word finding abilities. Problems with control of the autonomic nervous system results in palpitations, sweating episodes and symptoms associated with low blood pressure/postural hypotension (e.g. fainting).

Some symptoms which suggest on-going abnormalities in immune system function include sore throats, enlarged glands, joint pains, headaches, problems with temperature control and intermittent flu-like feelings.

Other symptoms which frequently occur in ME/CFS include sleep disturbances (often increased requirements at the onset followed by an inability to maintain a full night’s sleep), alcohol intolerance (a very characteristic feature, particularly in the early period of illness) and irritable bowel symptomatology.

Some people also develop emotional lability or mood swings and features of clinical depression as time goes on.

Besides these more obvious and wide-spread symptoms there are also myriad "minor" ones. Not everyone experiences all of them and often they are not mentioned when patients describe their illness; however there is often very visible relief when they find others, too, have similar experiences.

ME/CFS symptoms tend to fluctuate in severity throughout the course of a day and people often report that they have both 'good days' and 'bad days', although the term 'good' is often used in a relative sense. The illness almost always results in a severe reduction in a person’s ability to cope with all aspects of normal daily living (i.e. social and sporting activities, employment, household tasks). Relapses or exacerbations are often precipitated by infections, excessive physical or mental stress, general anaesthetics and surgical operations, and extremes of temperature.

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