Patients who fulfilled inclusion criteria and consented to participate received either treatment as usual at the clinic, or were enrolled in a group-therapy based intervention, the multi-disciplinary pain intervention, led by a Medical doctor, physiotherapist and psychologist at the clinic.

The Department of Pain Management and Research at Oslo University Hospital is Norway's largest outpatient pain clinic. It treats around one thousand new patients annually suffering from chronic pain of all aetiologies. Many of the patients receive interdisciplinary care based on a biopsychosocial framework, by at least two health care providers (physician, physiotherapist, psychologist, occupational therapist and/or nurse).

Participants were recruited through referrals from primary or secondary care, which were identified by an established interdisciplinary team. Patients found potentially eligible for the study were contacted by the research team and consulted with a research physician. Patients who fulfilled the inclusion criteria were invited to participate in the study and signed a consent form. Patients who declined to participate and non-eligible patients received usual care.

Inclusion criteria were 1) age 18 or more; 2) pain in at least three out of five bodily regions (defined as the four quadrants and axially); 3) pain persisting three months or more; 4) the patient was able to give informed consent and benefit from group therapy (including sufficient cognitive capacity and language skills); 5) no other medical condition could better explain the symptoms.

Subjects were randomised either to treatment as usual or a group-based multidisciplinary pain intervention (the MUD-PI).

Inclusion Criteria:

• Age 18 or more
• Pain in at least three out of five bodily regions (defined as the four quadrants and axially)
• Pain persisting three months or more
• The patient was able to give informed consent and benefit from group therapy (including sufficient cognitive capacity and language skills
• No other medical condition could better explain the symptoms.