The purpose of this study is to investigate the effect of fecal microbiota transplantation (FMT) on the symptoms of irritable bowel syndrome (IBS) patients.

Inclusion criteria for patients

1. Signed informed consent
2. Fulfilled Rome III diagnostic criteria for IBS, and frequency of IBS pain or discomfort for at least 2 days a week in the last 12 weeks
3. Age: 18-65 years

Exclusion criteria for patients

1. High proportion of butyrate-producing microbiota in fecal samples
2. Known organic gastrointestinal disease (e.g. IBD)
3. Previous complicated gastrointestinal surgery
4. Non-gastrointestinal malignancy
5. Dementia, severe depression, major psychiatric disorder, or other incapacity for adequate cooperation
6. Females who are pregnant or breast-feeding
7. Severe endometriosis
8. Antimicrobial treatment 4 weeks prior to first screening visit
9. Antimicrobial prophylaxis (eg. acne, urinary tract infection)
10. Regular consumption of probiotic products 4 weeks prior to randomization
11. Recently (within the last 3 months) diagnosed lactose intolerance
12. Celiac disease
13. Abuse of alcohol or drugs
14. Any clinically significant disease/condition which in the investigator's opinion could interfere with the results of the trial

Inclusion criteria for donors

1. Signed informed consent
2. High-butyrate producing microbiota in fecal samples
3. Age: 18-65 years

Exclusion criteria for donors

1. Known organic gastrointestinal disease (e.g. IBD, IBS, chronic diarrhea or constipation)
2. Gastrointestinal malignancy or polyposis
3. History of major gastrointestinal surgery (e.g. gastric bypass)
4. Eosinophilic disorders of the gastrointestinal tract
5. Current communicable disease (e.g. upper respiratory tract infection)
6. Known or high risk of infectious diseases such as HIV, hepatitis A, B or C
7. Non-gastrointestinal malignancy
8. Dementia, severe depression, major psychiatric disorder, or other incapacity for adequate cooperation
9. Comorbidities such as metabolic syndrome, autoimmune diseases, allergies
10. Chronic pain syndromes (eg. chronic fatigue syndrome, fibromyalgia)
11. Severe or morbid obesity
12. Use of immunosuppressive or chemotherapy agents
13. Antimicrobial treatment or prophylaxis within the last 6 months
14. Females who are pregnant or breast-feeding
15. Known clinically significant abnormal laboratory values
16. Participation in high-risk sexual behaviors
17. Abuse of alcohol or drugs
18. Tattoo or body piercing within the last 6 months
19. Travel to areas with endemic diarrhea during the last 3 months
20. Positive stool testing for C. difficile toxin, ova, parasites, Giarda antigen, cryptosporidium antigen