CFS/ME participants:

General practitioners recruit participants from the local area, by posters at the doctors' offices. In addition the study has a facebook site, named "the COMEBACK study", where interested CFS/ME subjects can submit their interest to be assessed for participation. After a telephone screening of potential participants by the Canada Criteria and CFS/ME severity rating, eligible subjects will be referred to department of physical medicine and rehabilitation UNN Harstad (FYSMED) and re-assessed. During this screening process the investigators will keep a track record of screening failures noting reason for failure. Participants will have a physical exam and necessary workup including blood, fecal and urine tests to exclude differential diagnosis according to the Norwegian National Guidelines for Assessment of CFS/ME. Participants receive information about the study and give their written consent. Subjects earlier diagnosed with CFS/ME at FYSMED will undertake the same re-assessment.

During the work up, participants will do the Fatigue Severity Scale, Hospital Anxiety and Depression scale, SF 36, Modified DePaul Questionnaire, the Rome IV criteria for irritable bowel syndrome, and the "Repeatable Battery for the Assessment of Neuropsychological Status" test (RBANS).

Donors are recruited informally from the local high schools. Donors are included and screened according to the European Consensus Guidelines from 2017. The full screening will be undertaken before the first feces donation and every 4th week. The inclusion and screening will be performed at the department of physical medicine and rehabilitation UNN Harstad. The investigators will keep a track record of screening failures noting reason for failure.

Participants receive FMT at the gastroenterology outpatient clinic at University Hospital of North Norway Harstad, Norway. No antibiotics are given prior to the intervention. The participants must do a bowel lavage using Sodiumpicosulphate/Magnesiumcitrate (Picoprep, Ferring) before intervention. The treatment will be administered by enema. Active treatment will be pre-processed frozen donor feces. Placebo will be the participant's own feces processed and frozen during the study inclusion. After the intervention, the participants have no restrictions on activity level and are asked to keep an unchanged diet without introduction of any new food supplements or probiotics in the follow up period. To keep track of change in diet investigators ask participants to do a food frequency questionnaire before the FMT and at 3 and 12 months after the intervention. Use of antibiotics, food supplement and use of medications will also be recorded.

The treatment will take place in blocks of four consecutive participants per day. A data engineer at the Department of Clinical Research at the University Hospital of North Norway, Tromsø (UNN,Tromsø) creates the allocation sequence using the REDCap software. The treatment is randomized on donor and placebo in fixed blocks of 4 with 2 active (1 donor A and 1 donor B) and 2 placebo. Block allocation will be stratified on gender.

A stratification on donor and gender will be performed by assigning full blocks of male and female participants. In the two active slots in each block of four, one active slot will be used for donor A and one for donor B. The stratification of gender reflects the higher incidence of CFS/ME in women.

Allocation is done in solitude in a closed room with no transparency, only containing a freezer with the active transplants (tagged by donor batch ID) and the placebo transplants (tagged by screening number). Before allocation of treatment, an investigator places the FMT-placebos on a table in the room. A minimum of four participants is allocated to treatment each time. The allocator can then enter the room as the researcher placing all the placebos leaves the room. The allocator will access the randomization sequence when entering participants screening number on the REDCap software using a computer in the same room. The allocator will be the only person involved in the study that can access the randomization program at the REDCap software. If a screening number is randomized to active treatment, the allocator removes the tag from the placebo and places it on a donor FMT treatment instead. All unused placebo transplants will be disposed immediately. When finished, the allocator places the allocated treatment in a box in a designated freezer. The allocator will build a key file matching the active treatment to the donor batch id by updating a key file on paper and store it in a safe not accessible to any others. In addition the allocator will write the corresponding patient screeningnumber on tags from the used donor batch and keep them as backup in the same safe. This will allow for tracking of each individual donor batch to a corresponding participant at the end of trial when all follow up is complete.