Background and aim: Evidence on the course of gambling disorder (GD) in clients seeking help from outpatient addiction care facilities is sparse. To close this knowledge gap, this longitudinal one-armed cohort study portrays the development of GD in help-seeking clients over a 3-year timeframe. Methods: We investigated changes in severity of GD as well as in gambling frequency and intensity in 145 gamblers in outpatient treatment in Bavaria using generalized estimation equations (GEEs). To investigate potentially different trajectories between study participants with and without migration background (MB), additional analyses were applied with timepmigration interaction. All analyses were adjusted for age, gender, education, electronic gambling machine (EGM) gambling, MB, GD, related help sought before and treatment status. Results: Within the entire study population, improvements in severity of GD (reduction of 39.2%), gambling intensity (reduction of 75.6%) and gambling frequency (reduction of 77.0%) were observed between baseline and 36 months of follow-up. The declines were most pronounced between baseline and follow-up 1 and stabilized thereafter. Participants with MB improved consistently less than participants without MB. Discussion and conclusion: Our study suggests that severity of GD and gambling patterns improve in the context of outpatient treatment. The beneficial results furthermore persist for 36 months after treatment termination. As clients with MB seem to profit less than clients without MB, improvements in outpatient gambling services to the specific needs of this clientele are required. Kulcsszavak: gambling disorder, outpatient addiction care, longitudinal study, migration background