Background and Aims: Gambling-related cognitive distortions (GRCD) sustain gambling behaviors despite adverse consequences. While previous studies have shown an association between GRCD and gambling disorder (GD) severity, few have conducted causal analyses. We aimed to examine temporal changes in GRCD among treatment-seeking patients and to investigate whether GRCD predicts subsequent gambling severity. Methods: This one-year follow-up study was conducted in collaboration with 20 addictionspecialized medical institutions. The participants were 100 male patients diagnosed with GD (mean age: 37.2+-8.5). We also examined the differences in GRCD improvement based on gambling relapse after treatment initiation. GRCD were assessed using the Gambling-Related Cognitions Scale (GRCS), and gambling severity was measured using the Problem Gambling Severity Index (PGSI). Gambling engaement was assessed using a self-report questionnaire. Temporal changes were analyzed using repeated-measures ANOVA, and the causal relationship between the GRCD and the PGSI was examined using a cross-lagged panel model (CLPM). Results: Overall, GRCD significantly decreased within 6 months of treatment initiation and then stabilized. The abstinence group had significantly lower GRCD scores than the non-abstinence group at all the timepoints. CLPM identified Perceived Inability to Stop Gambling (IS) as the only GRCD subscale that significantly predicted PGSI scores at 12 months. Discussion: This study demonstrates that GRCD predict the severity of subsequent gambling disorders. In particular, IS has been identified as a critical target for interventions. Conclusions: These findings provide valuable evidence of a causal relationship between GRCD and gambling severity and underscore the importance of targeting IS in treatment.  Kulcsszavak: gambling disorder, cognitive distortions, longitudinal study, treatment-seeking patients, gambling severity, causal analysis