Mindfulness Meditation and Self-Monitoring Reduced Maladaptive Daydreaming Symptoms: A Randomized Controlled Trial of a Brief Self-Guided Web-Based Program Oren Herscu 1 , Eli Somer 1 , Asaf Federman 2 , and Nirit Soffer-Dudek 3 1 School of Social Work, University of Haifa 2 Baruch Ivcher School of Psychology, Reichman University 3 Department of Psychology, Ben-Gurion University of the Negev Objective: Maladaptive daydreaming (MD) is a compulsive form of daydreaming that causes distress and functional impairment. We present the first treatment trial for MD. Method: We tested the effectiveness of an eight-session internet-based self-help training for mindfulness and self-monitoring and compared three groups across three measurement points in time. A sample of 557 people was randomly assigned. A total of 353 participants (age M[SD] = 28.3[10.5], 76% female, 77% unmarried) completed our program: full- intervention group (n = 114, psychoeducation + motivation enhancement + mindfulness + self-monitor- ing), partial-intervention group (n = 125, identical excluding self-monitoring), and waiting-list group (n = 125, internet-based support as usual). Results: All MD measures assessing daydreaming pathology, daydreaming frequency, and life functioning showed significant improvement with a large effect size (ES) from baseline to posttreatment in both intervention groups, whereas the wait-list group showed no significant improvement, MD: F(3, 349) = 35.76, p < .0001, η 2 = 0.24; frequency: F(3, 349) = 32.06, p < .001, η 2 = 0.22; functioning: F(3, 349) = 20.43, p < .001, η 2 = 0.15. Mindfulness with self-monitoring training for MD was superior to mindfulness alone in the short term, but they both were equally efficient in the long term. Both interventions were superior to relying on internet-based support forums only. The clinically significant improvement rate of mindfulness with self-monitoring training was 24%, while the reliable improvement rate reached 39%. At the 6-month follow-up, achievements were maintained. Conclusions: A brief internet-based intervention program comprising mindfulness meditation and self- monitoring facilitated recovery or improvement in many individuals with MD. What is the public health significance of this article? Maladaptive daydreaming (MD) is a distinct yet unfamiliar mental health problem. Given the lack of available treatment protocols, the implications of an effective intervention for clinical practice, public policy, and the well-being of innumerable suffering individuals are highly significant. Our article shows that a brief internet-based program involving mindfulness training and self-monitoring can improve various mental health indices of individuals coping with MD. Keywords: maladaptive daydreaming, immersive daydreaming, RCT, dissociation, treatment Supplemental materials: https://doi.org/10.1037/ccp0000790.supp Maladaptive daydreaming (MD) is an immersive form of compul- sive daydreaming that generates a sense of presence in fantasy, is rich in plot and characters, and is often activated during stereotypical movement (e.g., running, pacing, spinning, gesturing, or mouthing) and exposure to evocative music (Somer et al., 2016a, 2016b). Evidence suggests that MD might be a clinical syndrome rather than an isolated symptom. MD causes distress often associated with the time wasted in fantasy or the contrast between the idealized life in fantasy and reality. Although the activity may be rewarding in the short run, many individuals engaged in this mental activity report feeling negative affect such as shame and guilt afterward (Bigelsen & Schupak, 2011; Soffer-Dudek & Somer, 2018). MD can also impair academic, interpersonal, or professional functioning (Somer, 2002). It is considered an extreme form of dissociative absorption (Ross, 2018; This document is copyrighted by the American Psychological Association or one of its allied publishers. This article is intended solely for the personal use of the individual user and is not to be disseminated broadly. Oren Herscu https://orcid.org/0000-0001-5933-9074 Eli Somer https://orcid.org/0000-0002-1435-1763 Nirit Soffer-Dudek https://orcid.org/0000-0002-8778-798X Oren Herscu played lead role in data curation, formal analysis, investigation, methodology, project administration, software, visualization, and writing of original draft and equal role in conceptualization and writing of review and editing. Eli Somer played lead role in supervision, supporting role in method- ology and equal role in conceptualization, investigation, and writing of review and editing. Asaf Federman played supporting role in methodology and resources. Nirit Soffer-Dudek played supporting role in supervision and equal role in writing of review and editing. Correspondence concerning this article should be addressed to Oren Herscu, School of Social Work, University of Haifa, Abba Khoushy 199, Haifa, 3498838, Israel. Email: oren.herscu@gmail.com Journal of Consulting and Clinical Psychology © 2023 American Psychological Association ISSN: 0022-006X https://doi.org/10.1037/ccp0000790 1