Multiple sclerosis (MS) is defined as an idiopathic, inflammatory disease of the central nervous system characterized by demyelination and axonal loss. The common symptoms of these patients include numbness, weakness, vision problems, balance problems, vertigo, urinary urgency, fatigue, and depression (
1). MS is an autoimmune, multifactorial, and heterogeneous disease influenced by genetic and environmental factors. Due to high treatment costs, poor job expectations, and MS patients’ job dropout, this disease imposes significant costs on society (
2). MS is 3.13 times more common in women than in men (
3). A previous systematic review and meta-analysis estimated the prevalence of MS in Iran to be 29.3 per 100 000 people (
4). It has been shown that patients with MS may have different psychological disorders, including depression and anxiety (
5-
9). Depression and anxiety in MS patients are associated with reduced treatment adherence (
10) and quality of life (
11). The effectiveness of psychological therapies, such as cognitive-behavioral therapy (CBT), in reducing psychological problems has been shown by empirical research in MS patients (
12,
13). However, negative illness beliefs and distress may be realistic in specific medical conditions such as MS at certain times (
14). Accordingly, there has been an increase in the use of other psychotherapies, such as acceptance and commitment therapy (ACT), for chronic disorders, and some studies have shown the effectiveness of such therapies for MS (
14-
17). In contrast to CBT that aims to directly change negative beliefs, in ACT, the focus is on accepting thoughts and feelings and creating behaviors based on values (
18).
ACT is a third-wave behavioral therapy that creates psychological flexibility (
19). Psychological flexibility is a behavioral pattern defined based on 6 processes, decreasing or increasing the treatment model (
20). Cognitive fusion is one of these processes, playing a role in different psychological disorders, including depression and anxiety (
21,
22). In ACT, cognitive fusion is the dominance of cognitive events over one’s personal experience, inability to observe cognitive events from a different perspective, tendency toward showing emotional reactions to thoughts, overregulation of behavior under the dominance of cognitive events, attempt to control thoughts, over-analysis of situations, and making judgments about the content of one’s thoughts (
23). Given the importance of cognitive fusion in psychopathology, different researchers have examined this construct in MS patients. They have shown the role of cognitive fusion in psychological distress, such as anxiety and depression, as well as in the psychological well-being and quality of life of these patients (
24-
27). The Cognitive Fusion Questionnaire (CFQ) has recently been designed to assess this concept. The results showed that CFQ had a 1-factor structure and revealed its discriminant validity, sensitivity to treatment, stability over time, and reliability (
23). CFQ was translated and validated in Iran (
28). CFQ was designed to measure cognitive fusion in mental disorders and non-clinical samples, but some researchers (
29) have recently designed a Chronic Illness version of the CFQ referred to as CFQ-CI in inflammatory bowel disease (IBD).